A unified login for 2 separate instances and sites

We are looking to join forces with another Tessitura site with their own instance of tessitura and we want to make our sites look like they login only once.  We have been batting around a few ideas but we were wondering if any other organization have done anything similiar with multiple instances of Tessitura.  (Remember in order to process anything on either site they need to have a session with each individual site.)