If Taim were just Demandred puppet, one would reduce Taim's state as an interesting, major villian, a kind of "new Forsaken generation". As far as we know about his character and OP strength, Taim simply doesn't need Demandred as a big master. He can pull it off on his own. To mix Demandred and Taim together at one place makes simply zero sense story- and character-wise. It would also better illusrate what Ishy had in his mind by his offer to train Rand, and the example of Alviarin & Mesaana shows us how a Forsaken could train/teach another channeler with enough time.

If Taim was trained by Ishy in the past, though, and if Demandred has his own separated power bases, it would increase the sense of capability for Ishy, Taim AND Demandred.