New at this whole fondant thing. How much would I need to cover 2
12x18x3 cakes stacked on top of each other. Also, would I have to dowel them?
No, you wouldn't have to dowel them, but I would put buttercream or some type of filling in between to make a 6" + high cake - wow that it pretty tall! lol You may want to make each of them 2" tall so you end up with a 4"+ tall cake - looks tall even at just that high. But that's totally your call - you may have some specific reason for wanting it that high - so don't necessarily listen to me!
I have covered a full sheet (18x24x2) with about 4.5-5 lbs, but I just barely made it! If you have a MINIMUM of 5 lbs it should be enough to cover it. But since you are new to fondant, you need some room for error - I would have double that just to make sure - that way if you have to redo it you are not in a panic!