Like, one person does the inking, pencilling, and the other half does script, colours and lettering, something like that? In that case;
First: do we get along? Are they established in the community, do we know each other and can work together? Then, are we interested in telling the same types of stories, or at least stories that interest the both of us. When I collab I like to collab like you do: a back and forth with ideas. My current writer scripts then gets my feedback, and I send them my thumbnails, pencils, inks, colours when it's time to do all that. It's important because some writers will want to do stuff like cram tond of text into one bubble, or be specific with layouts that might cause issues.
Next, style cohesion: I have a distinct and specific natural style of inking, so if I was to inflict myself upon other people I'd look for, say, a colourist with a muted kind of style - nothing too fussy or detailed, or it'd fight with my pooling blacks. Someone who can work in flats or a nice subtle gradient, think Hellboy rather than Paper Girls, which is more detailed because the lines allow more room to breathe. We have to be on a wavelength.
But yeah. It should be a good friendship first, and you both should agree on stuff like "what happens if this makes money" and "what if one of us can't do this anymore". No need for a contract, but worth a chat just in case they feel VERY differently to you, and it won't be a surprise! Conversations like that save you sooooo much heartbreak in the long run, and they're awkward now but god, you'll be glad you talked about it when you need it.