Get position index of author

Hi,
Is there any option to get index position of assigned author?
For example, my every first author is main author others are just co-authors.
I want to create page dedicated to every author, where I would loop through all posts assigned to author but with two options:
a) loop by posts where author is main author (1st position in order)
b) loop by posts where author is just co-author (2nd and later positions)

Is such thing possible? Maybe we could save some meta field if author is on 1st position etc.?