This question is a hardy perennial and there are almost as many answers as accordionists

I, personally, prefer to begin at the beginning, both hands together, a bar (or phrase) at a time, very slowly. Once I have mastered a phrase and incorporated it in what has come before, I advance to the next phrase. It may take me weeks to work right through a piece, after which continued practice gradually brings it up to speed and consolidates muscle memory.
I find, when working both hands together, the bass clef acts as a framework or scaffold on which to erect the right hand notes.
When learning both hands separately, you still have the task of fitting the two together.
I also like to go through a piece very early noting the most practical fingering so I can concentrate on it.
Each to his own!