AI'd like to make a rectangular cake to feed about 20 people. The recipe I'd like to use is meant for a two-layer 9 inch round cake, but I want to convert it to something like a 13x9 that I can split and fill.

What would be the best way to go about it? Would I need to double the recipe and bake it in two separate pans? Or could I fill the pan with more batter and slice the cake horizontally after it's baked? I also have a 15x11 inch pan, could I bake a double recipe in that and split it in half crosswise and just stack the two halves?

Thanks!

A single layer 9 by 13 should be fine for 20 servings.  A recipe that makes 2 9 inch pans should also work for a single layer 9 by 13.  This is a most useful chart.  http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm  A double layer 9 by 13 serves 50 so then a single layer should serve 25 so that will work for your needs.   If you look in the center column it says how much batter you need for a single layer of cake.  So using that as a guide a two layer 9 inch cake would need around 11 cups of batter.  So we could assume that a recipe that is meant for two 9 inch cakes would make that much batter.  Looking down at the 9 by 13 cake it needs 7 cups of batter for a single layer.  Therefore your recipe should be more than enough for the single 9 by 13 that you can then split and fill as you would like to do.  Yes, there is a lot of assumptions but that is how I would try to figure out how it would work.