Imagine this: there are four players, each with different master. They decide to play 2on2 45ss game and each team wants to maximize their combined strength. These players disregard the possible hatreds and other story wise reasons that would make the team impossible.
so the question is: Which masters would be the greatest teams?
For example: Kirai and Molly. They both summon/aid/control spirits, both prefer to stay behind the front row and over all play well together. Their Minions, Henchmen and Enforcers mold extremely well together with Adversary and Reactivates as well as the pushes. While Seishins could make rogue necromancy, Izamu or any other harder character stay alive lot longer.
Edit: As for other great combination, MCMourning(res) and Brewmaster(either). Both of these masters are extremely poison heavy and could greatly benefit of each other.