if you can manage your time correctly yes. im currently doing A level maths privately over 1 year alongside a BTEC lvl 3. My advice is when learning the content keep going over previous topics that you have done so you don't forget them, as you wont have a teacher setting you homework on stuff like that. Also if you are doing edexcel maths watch a youtuber called Bicen Maths, he goes through the whole specification in order and its great. and try and do all your btec assignment work inside your college so you have more time to focus on the a levels outside of college. Also maths a level isnt hard if you enjoy it.