Two moles would be ridiculous, and given that all the Forsaken had decades of atrocities behind them, the whole idea of a mole amongst them makes no sense. One of them repenting and trying to do good... maybe that could work. One of them always being good, but part of an organization that killed millions makes no sense.
And a mole would just replicate Verin's story, which was both more believable and more interesting to me anyway.
But I would say that the way Brandon wrote Demandred it was obvious that he still cared for those around him, he was just vastly more wrapped up in his hatred of LTT. It would have been a plausible story to see Demandred flip sides. But yeah, it would take away from Verin's much more compelling story of Black Ajah Mole!