I just came across something quite interesting from socionics.com:
In order for Identical relations to last, one partner has to take a role of the Dual as if it was a Duality relationship. Usually it happens naturally. If the two are introverts, one often subconsciously attempts to take care of the extroverted side of things, if the two are thinking types, then one would try to fill the resulting emotional void etc. Different backgrounds and function developments of Identical partners could help in this case, however as with any other relationship there has to be a driving force behind it to keep it going.
When I'm around my INTp cousin, I believe that I turn into a sub-ENFp and he turns into a sub-ISTp.