No, there's never been a confirmation that Demandred is using an alter ego.

Jordan just said (before KOD came out) that we had never seen Demandred in the series so far except as Demandred.

Personally, I think Demandred isn't using an alias. I think he sticks to the tactics described by Sammael, that is he is pushing proxies on the board, while he stays hidden, and his agents are DF, so he doesn't hide from them who he is (like these DF Asha'man who have been given orders by him).

I think the BT is just a front for Demandred's real army. I believe Taim's obsession with recruiting hides the fact his cronies have actually used these parties, all those that included only cronies of Taim, as a front to get most male DF tested for the ability, and most of those who have tested positive have not returned to the BT but have been sent to Demandred's hidden lair. The game is nearing the finish line now. I think Demandred has recently provided extra men to Taim, so he could eliminate the few lightsiders remaining at the BT and their families. I believe we've seen some of these men recently, they're the new recruits who gain their strength and learn way too fast.

Honestly, I don't think it's such a big mystery that Demandred is behind the Black Tower but not there, and acts through Taim. As far as I'm concerned the cat's out of the bag since WH, when we learned from those DF Asha'man who tried to kill Rand with Osan'gar that Demandred wasn't pleased, ordered them not to get caught again, and to tell no one about their orders, not even Taim. Why on earth would Demandred care? It's not because he was involved in the plan to kill Rand in Cairhien, that was Osan'gar (and it seems Demandred never knew Osan'gar was Dashiva and an Asha'man). So why then? Obviously, Demandred would have been really pissed off that suspicions of treachery falls on the BT, if it's his front to recruit men openly... And then there's the question of who did keep Taim so paranoid about Rand's safety. Demandred wants to kill him himself.

The fact Aes Sedai have been turned to the Shadow is further evidence not only there's a Forsaken involved, but one who thinks like a general and has immediately seen the great danger there was if 47 Asha'man actually joined the WT, opening the way for all lightside channelers to fight with mixed gender circles in TG. The plan in place is obviously to delay the parties sent there, without raising alarm bells, until TG starts and to turn the sisters to the Shadow one after the other in the meantime (which took the help of 13 BA hidden wherever this is done nearby, and the help of a Forsaken to get 13 Fades to collaborate, and likely to explain how this is all done..). They've been turned not to act as spies, but to supplement the ranks of Demandred's army and provide it with mixed gender capabilities to counter Rand's newfound ones (because, the way Mesaana's plans had been going, most of the BA were stuck in the Tower. Demandred needed to train some women to fight.). Demandred has started planning to get women trained immediately after he saw Rand's using mixed gender circles at the cleansing. Rand then provided him with the opportunity to do so (again, as he had done by allowing Taim to recruit actively, not just passively) with his offer to Egwene to bond 47 men.

So I think Taim's group is merely a kind of infiltrated battalion of Demandred's larger army. As soon as Taim's done cleaning up all lightsiders, Demandred will move his Tarmon Gai'don HQ to the BT, under the dreamspike. His first move will be to capture the city of Caemlyn, use its population as food for the Shadowspawn armies he'll settle there, and to secure the Gateway in the city through which he will have these armies come south of the Blight, and through which he will be able to dispatch them to battle.

Demandred would never be satisfied with seizing control of a group or nations, of using non-channellers in his army to face Rand. It's a large following of channellers backed up with huge amounts of Shadowspawn he sought to assemble, an army of the kind he lead in the WOS after he turned to the Shadow. He's let the other generals of the Forsaken amuse themselves with building little powerbases in the open and fall to Rand one after the other. He's gone for the big price, and the war that counts: TG.