I am fine with two POVs and that's my current project. It alternates between chapters, and I too am trying to balance them. I feel your pain - because one of my characters is more interesting than the other.
My advice (to myself and) to you is to spend more time with your other character. And I pair this advice with : heavily edit out anything that doesn't need to be there. From the whole piece. Make it sharp.
So, get to know the secondary character better, just start working with that character in your mind or whatever, maybe hold an interview with that character on paper, ask them endless questions, get into their head, until you know their motivations and personality as well as you know the MCs.
And then keep editing to make it tighter.
I think the end product will be more satisfying to you.
Also, I have read one book where a few chapters near the end were thrown in from a 3rd POV. Yes, it was jarring. So you can do a few chapters like that, but it will probably be jarring. We sign up for a certain experience at the beginning of a book - You are giving us a contract, as Mark Baker sometimes says. Once I've signed the contract, I don't want the terms to change.