The gemara derives from the 3x repetition of the issur of basar b’chalav in the torah that there are 3 prohibitions associated with the issur: cooking, eating, and deriving benefit from basar b’chalav. If one cooks neveilah meat with milk, since the meat is already assur to eat because of the issur neveilah, we apply the rule of “ain issur chal al issur” (you can’t prohibit something already prohibited) and the issur of eating basar b’chalav has no effect. The Rambam (Peirush haMishnayos, Krisus) adds what he calls a “nekudah nifla’ah”, an amazing point. Not only does the issur of eating basar b’chalav not apply to this neveilah cooked in milk, but according to the Rambam the issur hana’ah does not apply either. Even though neveilah is not assur b’hana’ah so it would seem “ain issur chal al issur” does not apply, since the issur hana’ah is derived from the same pasuk as the issur achilah, the two always go hand in hand. Hana’ah is just an extended form of the issur of eating.
What is the din regarding drinking the milk in this neveilah b’chalav mixture? Since the milk is not neveilah and there is no issue of "ain issur chal al issur", one might conclude that the milk does become prohibited because of basar b’chalav. However, many achronim assume that the milk itself does not become assur either. The Chazon Ish (Y.D. 22) offers a fascinating hesber (which I’m not sure I fully grasp) which sounds very much like the Rogatchover’s distinction between “harkavah mizgit” and “harkavah shichnit”. A composite can consist of multiple items simply located in proximity, or can consist of a synthesis into a new whole. Basar b’chalav according to C. I. is a harkavah mizgit – both food items must form a new synthesis, a cheftza in which both parts adopt a new identity of issur basar b’chalav. Since in our case the meat cannot attain the status of issur basar b’chalav because it is neveilah, the milk also will never become assur because this new harkavah mizgit is missing a necessary component part of the synthesis. What stands out in this case is that the harkavah mizgit is being prevented not by the physical absence of meat, but by the meat being unable to change its halachic status to a different state – to me that sounds like quite a chiddush!