do you have to put dowels on a 9 X 13 double layer cake with fudge in the middle and butter creamed cake ?

I wouldn't.

Doweling is ONLY needed if there are multipule tiers above it. Example: you make two 10x4" cakes and want to put one on top of the other then you would use dowels to support the weight of the cake on top of the other. Anything over 4 or 5" above needs support.

thank you !!