If you don't feel like working on one of them at any given moment, do you feel like working on the other? Having two ideas can be a lot more forgiving in that way than just having one.

If they are complex ideas can you break them down into a "buffet" of subtasks? The idea is that hopefully on any given day/evening at least one of those small tasks will appeal to you. Doing something peripherally helpful is at least better than doing nothing helpful... as long as you have the energy to tackle the hard stuff every few days.