It's possible that the 2 volumes will be called A Memory of Light part 1 and A Memory of Light part 2, but I doubt it will happen because it emphasizes too much for the average reader (not the dedicated fans who already know it) that it's half of a book, making them focus on that instead of enjoying it for what it is. It's possible that they'll be called A Memory of Light: Some Subtitle and A Memory of Light: Some Other Subtitle; this seems more likely, but it adds a layer of complexity that could be confusing to readers and still somewhat brings to the fore the idea that there's more than one book of A Memory of Light.
I think the most likely result is the two volumes having completely different names. A Memory of Light could be the name given to the first part and the second part could be called something else, but this seems less appropriate to me because I've been assuming A Memory of Light's title was chosen because of last book considerations (I can't help but think of Babylon 5's last episode, Sleeping in Light)—it just feels like a last book title to me, doesn't it to you? Still, this could potentially cause problems with booksellers who are already taking preorders for A Memory of Light—would the orders be transferred to the first volume not called A Memory of Light, or kept for the second volume which will be called that? The first option makes the most sense except some people would inevitably say, "Hey, this isn't the book I ordered!" Also, Tor has already started its marketing with those widgets, you know?
Ultimately I believe we won't have to wait long to find out which of these situations happen (I think these 4 options are the only possibilities at this point and there's no chance it will be released as one volume). I am confident that we will hear the final word on the title of the first volume, and its release date, in April at JordanCon, if not slightly before that (I don't see a good reason to announce anything before JordanCon, though; JordanCon will be a good hype vehicle). If the book title is announced as A Memory of Light then, it's not going to change to something else by October/November because the timetable would screw up the book's marketing.
I don't have any inside information on this, but publishing is my business and these guesses are as educated as anyone can expect.