Demandred might have used Kisman before, like other Forsaken occasionally visit DF, even if they don't have their power base at the location of this DF (e.g. Be'lal snatches Alviarin in the WT once). We also know that most Forsaken have DF networks. That doesn't mean that Demandred is involved with the BT itself. Besides, if Demandred controls Taim, why should he care, if Taim learns of his order, which is anyway the same that Taim gave.

Furthermore, I think that actually Aran'gar posed as Demandred in WH to send the DF Asha'man to kill Rand. That would fit with Aran'gar's thought in KoD, after it is revealed that another Forsaken apparently posed as Sammael, that someone was playing her own game now, possibly Demandred: "[...] Or an attempt to hide being the one who had worn Sammael as a disguise. She would like very much to know who had begun playing her own game. Or whether Sammael really was alive."

And with the fact that Demandred wants to kill Rand personally.