After a game last night I was wondering who should be backing up 2nd base....ground ball hit down first baseline, 1st baseman fielded and stepped on base then threw to 2nd for a double play....wild throw, on line but way too high...went to outfield, left center, so should the center fielder or left fielder have backed this up?

Both... is the best answer; but I'm going to say the leftfielder should have the more 'natural' path to this play. the ball was hit to the 1st base side so that should move him a few steps right there; as he sees the play developing in front of him he should be there to back it up....
of course if they were playing the hitter around to pull to Left field then it's an easy back up for the centerfielder.

The whole thing is that both of them should have seen the play developing and known that they needed to move.

This should be the left fielders priority to get to this ball.