If they are OK with CBT then how about tying a strap around each of their scrotums, connect the straps with a rope that's maybe 6 feet long.
Then they have to play "tug of war" with the rope.
The loser has to perform a forfeit.
Licks the other guy's ass.
Licks the other guy's cock (not to orgasm)
Takes some number of strokes of a cane, or belt, etc.
They play "best of 5" (or best of 9, however long you want to stretch it out). Give them a few minutes rest between contests, and play other games. Maybe the loser of that round is humiliated verbally for a while - try to get him riled up so he performs better next time.
The winner of the whole contest gets to come, or whatever.