There was a long discussion on this a while back, which (in my opinion) was inconclusive.
The and/or concept that Paul describes is a nice way of describing how you, as a person, interact with the controls. However, I (and a few others) am convinced that mathematically, you can get exactly the same range of possible pickup combinations with either two volumes, or a pan. They are just two different ways of adjusting the relative contributions of two pickups.
I don't believe anyone offered a satisfying explanation of *why* they would end up sounding different. (There were a few distractions, such as a couple of cases where the pan control wasn't implemented quite right and gave uneven results, but that's a different issue.)
I remain unconvinced that they fundamentally sound different, and personally prefer a pan. In fact, if I did have separate volumes, then I would also want to have a master volume.
To me it's a matter of how you need/want to use the controls, and there are good reasons for either approach, depending on your style. I really don't believe there is any reason to believe you would be sacrificing the Alembic sound, so if the pan is what fits for you, then go for it.
-Bob