You'll have to settle for 2 taps. Camera button then video button. I know, i know it's tuff. But there's nothing that can be done. My fingers are bleeding nubs. I'm going back to ios!!
I do not know how you can do it.
However there is a chance that it can be done in the future if you write your idea on microsofts user voice Feature Suggestions: Hot (20393 ideas)
Who knows, maybe people vote for your idea and it gets highest priority.