Making a cake this weekend (actually for my own son's baptismal reception--so I am not as nervous as I would be if it were for a client) and wanted to try the double stacked look. We are having 118 adults/25 children at a catering venue. My original plan/sketch was for a 12", 10", double stacked 8", 6" round-tiered cake. 

My first question is: is sticking with the 8" tier as double stacked an aesthetically pleasing and more importantly, structurally sound design? Or should I double stack the 10" tier instead? 

Secondly, this is the first time I will be double stacking a tiered cake. In my mind, I am thinking I just need to support the tiers with a long, wooden support down the center as well as dowels in each of the 12", 10" and 8" tiers. I've only done the center support for one other cake and never had a problem with stacking remaining three to four-tier cakes without center support just individual dowels. I imagine however, that when double stacking a tier it would be better to insert a center support? 

Any advice would be greatly appreciated! Thanks! 

Doing a double stacked 8" tier is fine, both structurally and aesthetically, just make sure you support the double stack accordingly, assuming that this tier will be 2- 8"x4" you will need to put a board and dowels between them just like the other tiers. If you are transporting it stacked definately use a centre dowel.  Here is a picture of a wedding cake I did with the same size tiers with the addition of a sphere on top of the 6", i didn't have a centre dowel as i assembled the cake at the reception due to the 2 1/2 hour drive. hth.

How beautiful!

I've always transported cakes assembled, but without a center dowel, as venues have never been more than a 20-minute or so drive away. Only the first stacked cake I ever made had a center dowel as it was my first time creating such a cake and I was extremely nervous to say the least. Since then, I have never used a center dowel on simply stacked cakes, be it three or four tiers.  But when considering doing this double stacked design, I automatically thought I should add a center support since the 8" tier will make the cake fairly top heavy. 

Thanks for the feedback! 

A central dowel rod doesn't "support" anything.  It may help to prevent tiers from sliding apart from one another during transport.  Regardless of how heavy tiers are, a stick poked through the middle can't provide support nor prevent collapse -- only individual dowels below boards/plates can provide support.  Dowels without boards simply pierce through cake.
 

My apologies for using incorrect terminology within part of my post. 

AVery pretty!