I need to bake a cake for a friend and she gave me a picture/size of the cake she wants...and it is a rectangular cake that is about 16 inches by 12 inches...I thought a half sheet cake pan would be good....but unfortunately I dont have that....she needs to feed about 35 ppl.
Can I use any other rectangular pan and join the pieces with icing to create the size and put BC on top to have one solid recntagular block???

How high will be the cake? Double layer, meaning 4" high? For 35 people half sheet pan is too large - 9" x 13" still feeds 40 ppll, the 16" x 12" feeds 60.

But of course you can "glue" cakes with buttercream together - you need very stable cake board underneath, but otherwise it isn't a problem.