Please help the last time I tried this is was a disaster!!!

I need to stack two 7"x11" cakes on there end to make a 3D TV.  I don't want them to topple over. 

As well the client would prefer buttercream but that makes me even more nervous as fondant will add more stability.  Any one tried in buttercream?

Any advise would be greatly appreciated.

Well no wonder, that's a recipe for disaster. The way to do it would be stack normal layers of cake and carve it to get the shape you want. You can't really take a stacked and filled tier and just turn it on it's side. Just not a good idea. 

Oh, and if a cake needs/should be done in fondant, *they* don't get to tell you otherwise. You tell them how it will be done, or they can seek out the services of someone who wants to get into all that mess. Oh here, check out the Toy Story cake in this link. Methinks this was the work of a decorator that couldn't say no to the customer who insisted "no fondant." 

http://www.cakewrecks.com/home/2013/1/3/great-expectations.html

about every four inches of height you need dowel

within that four inches you either are using nice firm pound cake

or you have a two layer cake

here's one you tube video for some basics

also for a top heavy sculpture you wanna jam some dowel down through into a layer of styrofoam down under the bottom board

the structure of a sculpture, the boards and dowel, should just about stand on it's own without any cake in it--that's how secure you wanna make it--the cake just adds a bit of glue

I'm sure I have seen a purse cake post where they took 2 round cake, cut both ends, stood it up and then covered with icing and fondant..........I guess you can't do that with just any size???

I have no problem advising my client that it needs to be fondant. 
 

I did a 9 inch heart on it's side and iced it with cream cheese buttercream and I would never do that again. I did pound in a center post to keep it from tipping, but tried to keep it as a single piece.   Since it was for a home cake, it wasn't a problem or any thing,  Even though it was a dense pound cake, i just kept settling until there were wrinkles in the icing.  If I did it again, I would  cut it into 4" sideways cakes and reassemble it with an inside support system.   I can't get it to attach a pic, but it is on the second page of my pics.

yes, rearly,

bakeries and grocery stores do something like this routinely with no problems

cut 8 or 9 inch cakes in half and place both cut side down sandwiched together with icing

(look like half moon shapes)

to make an almost endless variety of cute efficiently produced sculptures

such as, rabbits, dogs, over the hill, purses, gym shoes, etc

so it fits the general rule of thumb to place board and dowel every  4 or five inches

in this case none is needed because it is not tall enough at 4 to 4.5 inches each

for the original question yes you need serious structure for what you propose as AZCouture advises