Are you using a stiff dam for your filling between layers?
sometimes the reason may be that you are not placing the top layer of cake exactly in line with the bottom layer after you've filled it. i also had this problem until i started properly realigning my cakes.
you may already know this but i'll mention one tip i use to ensure this realignment. as soon as you've split the cake, but before separating, insert 2 toothpicks in line with each other on the top and bottom half. after you've filled the bottom layer, place the top layer ensuring that the toothpicks are in line again. i find this gives a neat finish (almost like putting 2 pieces of a puzzle together) and makes sealing the join a little easier. hope this helps. good luck.sulia
I agree with Sulia! Aligning the cake so that you are putting the two layers back in exactly the same position is really important. There are all sorts of different methods you can employ (like the toothpick idea) such as cutting a small notch in both layers before separating.
It could also be about building a dam for your filling. I'm not sure if you described how you're noticing that it looks like two layeres--is the cake bulging out (lack of/insufficient dam) or are the layers clearly not in alignment from top to bottom because one of them was rotated or otherwise not placed back exactly in position?
I trim the sides of my cakes before icing. It makes a difference for me. I never use a stiffer icing, this works for some folks but no bakery I've ever worked for does this. I shave the sides of my cake so they are completely level -- it's like Spanx* for cake-- 
*modern day huggy underwear/girdles
I find once I fill the middle I lightly crumbcoat the entire cake and it seems to cut down on the centre line.
Definitely a stiff dam around your filling 1 inch from the edge of the cake so it has spreading room and so it doesn't come out the sides as it settles... Also letting the cake settle well enough before icing it. There was a thread not too long ago about letting it settle or settling it yourself (by pressing down until you see the dam reach the edge of the cake). I always had the line until I read this tip, especially when using this fillings in oh so hot texas.
Stiff dam.
Trim the outside crust.
Let it settle, or squish it - evenly...don't just push down with your hands...use a cookie sheet so you get even/level squishing.
Spackle.
Crumb coat.
Spackle,again if needed.
Chill and let it settle some more.
Oh, and don't forget to level the individual cakes before you even start...I'm wondering if you may have a domed top on one of the cakes, and that might be causing the line around the centre as well...??
When you fill your cake pans, overfill them a little more, sothat your sides bake up as high as the edge of the pan.
You can then trim the tops easily, for cake balls, yumm...
Or...
When I'm cooling my cakes, I flip the pan over onto the cooling rack, and compress the cake back into the pan, so it's level.. that way, no worries about trimming anything.. I use my pyrex measuring cup on top of the bottom of the pan for weighing it down.
HTH!! 
I always make sure to let my cake settle for several hours (even overnight) after it is filled to make sure that gravity does its thing before I ice it. Once it's all settled, I trim off any bulges that formed and I'm always left with a perfectly straight, smooth cake.
If you don't have time to have your cakes settle overnight, do this instead. Take a sheet of plastic or parchment paper and put it on top of the cake. Then take some books that are somewhat heavy but won't kill your cake, and set them on top for 3-5 hours. It works really well. I've been doing this for a wile now and it helps with bulging issues.
The best visual on avoiding the bulge: http://www.cakeboss.com/PreventBulging.aspx
When you pipe your dam, pipe it slightly inside the edge of the cake.... not right on the edge. Then when you put the top layer on and as the cake settles, if the dam is pushed out at all, it's pushed out TO the edge, not OUTSIDE the edge, which is contributor to the bulging thing.
I thought the OP was only using two layers...not splitting them
I also thought this. I read the toothpick advice a couple of times before I slapped myself on the forehead and said, "Oh! Now I get it!" 
Ooops to me! OP, the toothpick trick is good advice if you are torting the layers. 
indy that link goes to software
Just copy and paste the entire link into your browser - it will take you to a tutorial within the Cake Boss site.
indy that link goes to software
Just copy and paste the entire link into your browser - it will take you to a tutorial within the site.
Thanks, Dawn. not sure what I did different ... I know I've posted that link before and it worked ok. Gotta figure this out or I'll go nuts! 
indy that link goes to software
Just copy and paste the entire link into your browser - it will take you to a tutorial within the site.
Thanks, Dawn. not sure what I did different ... I know I've posted that link before and it worked ok. Gotta figure this out or I'll go nuts!
No prob - it's that hyperlink to cake boss that throws you off!
I don't use a stiff dam for my cakes - just SMBC. I recently used it as my dam on a very important cake on the advice of K8Memphis. It worked beautifully. I always trim the sides after filling and settling (I put a ceramic tile on top of my cake on top of parchment, in the fridge, if necessary to speed things along). Once the cake is settled where it will stay, I just position my knife, looking at it from above, and begin to cut off all excess bulges where I see them. A quick turn of my lazy susan to get a full picture of what the cake looks like, and voila! You're done! I hope this helps! Lots of luck on your future cakes!!
If you don't have time to have your cakes settle overnight, do this instead. Take a sheet of plastic or parchment paper and put it on top of the cake. Then take some books that are somewhat heavy but won't kill your cake, and set them on top for 3-5 hours. It works really well. I've been doing this for a wile now and it helps with bulging issues.
Oooh! I'll have to try this! Thanks for the tip.
sometimes it's just a matter of having thicker frosting on the outside...maybe you did yours too thin
I know what you mean about building it up. And most of my cake life I've done it this way. But invariably when I build it up in the trouble spot one more time and run my spatula over it to smooth it the brown spots appear again.
I could have made & smoothed upteen jillion more cakes for the time I've spent covering and smoothing that dang pesky brown area over and over and over. <insert sweat rolling off forehead smilie face>
So trimming the edges just ever so slightly but evenly and getting that all more smooth in the first place where there's no outstanding edges peaking through-- I mean even an eighth of an inch can kill you.
Works for me.
But also the tops of your cakes must be level--the crowns must be removed and they must be l.e.v.e.l or the icing will absolutley squish out just from the physics of it. Where one or two curved surfaces meet that are covered by a soft substance (icing of course) it will have to squish out just from gravity.
If only my bulge theory worked on my thunder thighs & cellulite like it does on my cakes. sigh
I don't use a dam at all, but I do trim the cakes so that the sides are smooth before crumb coating them. The only time that I have trouble with a "visual" on the center of the cake is when the icing is too thin on the cake. Trimming the sides so that the edges on the two layers are all even makes a big difference.
Here is a direct link to the CakeBoss 'Prevent Bulging' tutorial (debi, you didn't do anything wrong, it's just the embedded link for our trademarked name and web site that throws it off!)
http://tinyurl.com/pq9swv
Quote by @%username% on %date%
%body%