If you are only talking about an access point such as login with a link to another oxwall site then that is not as much work but i wonder why you would need that. What is your idea and the end gain?
What happens is that when a user registers on one site the information is passed via api to the other site and stored. This would have to include registration, user profile changes, user password changes, user email changes.
Plus you would have to either decide which site would be the primary passer of data or you would have to go back and forth both ways passing data between them. It could become quite a headache and quite a time management challenge to do this for a number of reasons. The actual gain you get may not be worth the upkeep and challenge in the end. Especially if Oxwall changes process then youll have to do more coding to include those changes in your api.