I can't point you in the direction of anything, but I don't know what "problems" it will cause.
You two should have a contract outlining your agreement as co-authors. You'll need to talk to a lawyer about that - I *imagine* the contract would be drawn up under either of your jurisdictions and state it is binding for worldwide rights (or something) but IANAL and have never had an occasion for such a thing.
Anybody from any country can sign a contract in another country. There's always a bit which says something to the effect of "by signing this I agree to be bound by the laws and ordinances of Narnia". In the event of a problem, you resolve it in the appropriate country.