Can the person that coborrows on a mortgage with me owner finance his half to me after we sign bank documents?

So I dont qualify for the house I want on my own. My girlfriends step dad is willing to coborrow a mortgage with me. I can afford the payments on my own but the banks dont think so. We haven't signed anything yet. I was wondering if he can coborrow with me and then right after begin owner financing his half of the mortgage to me. Where I end up paying for everything and being the owner.

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