As long as the second country is virtually a part of the homeland, basing is possible already. Some states become independent during gameplay, so you will have to be prepared to take your business elswhere. I donīt think that a mere association is valid for game rules, as it might be in RL, example would be EU here, where basing wonīt be liberated until the special "freedom of skies" treaty becomes valid somewhen in the late nineties. So itīs a try and error game and you could search for reliable evidence in case something is missing, but game canīt reflect all special treaty situations between single countries for the whole timeline. American carriers not only used Guam for basing, but also Tokyo,(evidently a relict right from WWII) which is not possible in this game.