So I'm attempting my first "boob" cake on top of a sheet cake and im worried that the boobs will be too heavy for the sheet cake beneath it... Any tips please??? :) 

Just dowel it if you're worried.  Put the boobs on small boards and 3 tea straws under each ;-)

I've done the preg mommy cakes and making the boobs out of rice krispie treats then covering them in melted choc to make them smooth keeps down the weight ... Esp if it's going to have fondant etc on it ... Hope this helps good luck can't wait to see a pic of your cake :)

Use cupcakes OR mini doll skirt pan to make the boobs.   No problem with them weighing too much.