When should you split a sponge cake in half - when cool or okay to cut when warm? Or is it better to bake two and sandwich them together with

Definitely wait to cool. You could bake two, but it will still require you to shave off a layer off of each top so that they are flat. In my opinion, it is easiest to bake one cake and split when it has cooled, just before you are ready to frost. Hope this helps!