The Ravenblack Grimoire

Yes and no. It depends what the powers entail. If you are trying to do two quests at once, no you can't. If, on the other hand, you are going to buy a non-quest power and a questing one, or two non-quest powers, then yes.

So, for instance, if I want to get Thievery 2 and Shadows 2 at the same time [both are at the Thieve's Guild, BTW], then I would surely be able to. However, trying to get Suction and Stamina at the same time will not work.