I copypasted OneLittleFonzie's comment from that thread:
"The 8 voice board sends half of it voices to output A and the others to B. If you don’t connect a plug to B, those 4 voiced get merged with the first 4 into output A. Regardless whether you use multi, no multi, stack or split. It is hardwired. Same for the expansion board. The first 4 go to A, the others to B, unless you don’t plug anything in the B output."
All 8 voices should go to output A, as long as you haven't connected anything to the B outputs.
If I remember correctly, you had to set this up in the global menu on the Prophet'08 (Output B: ON) but on the Rev2 it should be hardwired so that if nothing is connected to the B jacks, all voices SHOULD go to Output A.