I frequently read a lot of books, and I can spot shitty writing from the description and the first few pages.
This guy posts on a forum explaining that he can't figure out how to make a female character exist as a main character with a male character as equally important but they aren't Romantically involved. So you want a female character to be a shared main character (protagonist) with a male character but not have them be a romantic interest?
"You know, I don't have a romantic interest in the cashier at Walmart, but neither of us are particularity fun to read about either."
Simple: cut the fat, or make them siblings/related.
If you aren't going to have a romantic relationship, sexual tension, or familial dispute, why have opposed genders as main characters? Just focus on one and not have two opposed gender main characters. Make one a side character that shows up from time to time and do something and leave. Focus instead on one protagonist, and not two. Because when