That could actually work, but I get the feeling they'd do it like AMJ and have MJ fighting super-villains anyways lol.
But if they were to let MJ have MJ have her own stories without super-hero battles while Felicia is the one who does that, then it could work.
You don't put a supporting character's name on the main title like that, considering super-hero comics have the comic have the protagonist's name or codename in it (Or both), MJ having her name in it means she's sharing the title with Felicia.To be honest I feel like that would make it sound like MJ is a supporting character in a Black Cat book, so I would assume they would actually give her her own story than just making her function in Felicia's stories.
Of course, they could go for a "Black Cat and Mary Jane but Mary Jane is actually just a supporting character", but that's false advertisement, which's, nothing new for comic books, but I would rather they don't do that, still a possibility unfortunately.