Storywise it would also be pretty strange if two such similar men like Taim and Demandred are closely working together. I think Taim would be very hard to control anyway for a man like Demandred. It also doesn't make much sense then that Osan'gar was sent to the BT, or that Demandred doesn't know the AM on Rand's side (like Flinn). Furthermore it doesn't explain what Demandred did in the books before LoC. RJ also said that "those [Forsaken] with the largest worldly power bases would be rewarded most" by the DO. The idea that Demandred only managed to control Taim, his 100 cronies and a farm in Andor is quite pitiful in this context (especially considering that Taim is still acting on Rand's orders to provide him AM in TGS and that most of the other hundreds of AM are weapons on Rand's side). Besides that, several clues indicate that Demandred was behind the Dragonsworn at first and later additionally controlled in Murandy (plus maybe Tarabon). To control so many groups/nations would be quite a feat!

IMO, the best explanation is still that Taim was Ishy's disciple in the past (explaining his knowledge), was later used by SH (who also gave him the Seal) and is now directly supervised by Moridin and SH.