I'm wondering if having the light bulbs in the center back or opposite sides in the center, with fans directing the heat would be better.
Having both top down makes me feel like the top shelf would be very hot. At the very least one at the top and one centered, or at the bottom.
Factor in heat rises as well (depending on how much the cupboard traps heat.)
If your fans are very strong it may not matter but you would probably want to factor in redundancy-- what would happen if one fan broke? Instead of depending on them fully to not cook the eggs on the top shelf, spreading out where the heat centers are could help keep things even.
@BantyChooks has built several incubators and maybe she wouldn't mind taking a peek. Otherwise, you could peruse through our Incubators and Brooders articles and find lots of examples there.