It could be possible (depends on your college) but it does not make any sense. BTEC Art & Design will give you more than enough opportunity to build your portfolio - if you are doing the Extended Diploma you are basically supposed to work full-time on it (I'm sure it has Graphic Design option units as well, though not sure if your college offers them). If you want to do an A-Level alongside (which is a good idea), you should take a contrasting one that may still be useful for your degree choice, like English, Business or Computing or Maths, depending on your preferences and abilities.