I have a wedding cake due this weekend, where the bottom and top tiers are fake and the middle tier is real cake.

How do I stack them so they won't shift? Should I do a sharpened thick dowel down the center of the whole thing?

Also, how do I get fake burlap flowers to stick to the tiers? The flowers the bride gave me are quite flat on the back with no stems. I'm not 100% sure they'll stay put. I did a quick experiment over the weekend to see what would happen to the bettercream and the flowers while sitting directly in the sun in 95 degree F heat. They did just fine for the first hour, so maybe they'll be ok? The cake will be brought out of a refrigerated trailer about 30 minutes prior to being cut.

Perhaps I should hot glue the flat backed flowers to the faux burlap ribbon?

Thanks in advance for your ideas CC'ers 

I'd just use loops of tape to attach the real cake to the fake cake, but a dowel through the two works too. Don't dowel through the top one because that will make it hard to disassemble to cut.

If the fake flowers can be attached to the faux burlap with hot glue without having it touch the cake I'd do that.

Thank you @costumeczar  Great idea to use just tape to attach the cake board under the real cake to the top of the fake cake. How would I attach the fake top tier to the center tier which is real cake? 

Just stick it on with a little icing. You might want to put a plastic board or waxed paper between it and the real cake to keep any styrofoam bits off, but I wouldn't worry about that one. If it's stuck to the middle tier with icing it should stay put. If you're really nervous about it you can put a skewer through it and into the middle tier, then just tell the person who will cut the cake to lift the top tier straight up to remove it.

Ah! Ok. Wasn't entirely sure using the icing would make it stick enough so it won't slide. Those cake dummies are very light so I can see that working now. Yes, I planned on putting the styrofoam top tier onto a cake board just in case. Thanks for your help @costumeczar  !

The one time I had a dummy in the center it shifted a little durring transportation.  It made me so nervouse the entire time I had to pick up my sister to have someone ride with it in the back.  For me, the dowel is the way to go...just my $.02.

She is not putting a dummy in middle. The middle is the only real bit of cake. I assume the dummies will be covered in fondant? If so, then royal icing is the way to go to glue cake board to bottom dummy. Top dummy can be placed prior to taking cake out for serving. Four bamboo skewers stuck to the underside of top dummy (through cake card) will make it easy to sink into cake at last moment. What do you think? As for burlap and flowers, you can make your own mould out of siligum and use fondant or modelling chocolate to recreate edible version.

Of course you are correct, I didn't read closely enough. 

Thank you @Shockolata  I like the idea of the skewers. The tiers will have bettercream. I'm thinking I can just used double sided tape to attach the middle real cake to the bottom dummy. I might have to use a couple of cake boards to build up the middle since it may appear sunken with the bettercream on the bottom dummy. For the top dummy I'll use skewers and sink them just deep enough to go through it's cakeboard and into the top of the middle cake.

If it is just a matter of glueing the middle tier to the bottom,  I don't think that double sided tape will stick to polystyrene. You might have to use a glue gun. Best make a test run with some scrap polystyrene and a small cake board. Note that polystyrene can melt so apply the glue to the cake board (without a cake on top of course) and then stick that to the dummy.