Setup a company, both submit to the company and the company owns the copyright and submits to the agencies. other than that one person legally has to give ownership of their images to the other.
Like any partnership get it written up as who does what, what the terms are, income splits (ie 50/50? image tracking?), asset ownership, etc. also what happens when you have a disagrement and one walks out (which noone accounts for, but happens to the huge amount of partnerships and often destroys friendships)