Correct way to join two halves of sponge cake?
I've actually done some cake decorating (non-professional, but I did take a few classes), and I'll go with an option you didn't give:


Only bake one cake and split it in half (reduce the temperature of the oven, longer time, and if necessary, use cooling strips).You'll likely need taller pans for this -- you'll want light colored aluminum, 3" high for most general purposes.  (dark pans absorb heat too quickly, so the sides set before it's risen properly).  Level the cake (trim the domed-ness of the cake off), and stack the cake upside down, so you have the nice cake-pan edge on the top layer.


If the cake is too far domed, trimed it down some and save the removed bit.  Stack the cakes (again, upside down), crumb coat it, and take the removed cake, crumble it up into some icing, and then pipe that as a sort of filler around the bottom of the cake. to make up the gap.

If you're going to be filling the cake with something like a pudding or custard, stack the first layer like your answer C, pipe a ring of icing around the edge to hold it in, fill the custard in, then stack what had originally been the bottom of the cake (again, upsde down).  But make sure to make it thick enough in consistency, and thin enough in height -- I had an incident once where it formed a slip-plane, and the cake started ejecting the top layer off of slices.

To level the cake, you have two options -- they make special devices for doing it, which is basically a wire cutter on adjustable legs, that you can pull through the cake at a fixed height.  (also works for splitting the cake so you can then fill and stack it), but if you have a good long knife, a fairly steady hand, and a turn-table, you can:


put the cake on the turn table.
holding the knife steady, spin the cake and move the knife slowly in towards the center.


I find that a plastic cutting mat works pretty well to help get in there and take the layers of cake off after you've split them.

If you're going to be stacking cakes very tall, you'll want to use a pound cake recipe, or augment a boxed cake mix -- add in a box of instant pudding to firm up the resultant cake.