It reminds me of a situation on my own session. A guard's tower, filled with orcs, inside a female priest in plate mail battling with the orcs, the rest of the party outside. The necromancer was firing his sling through small window; weak shapeshifter held the door open, while the bowman was helping the priest inside with a valley of arrows.
I agree with DJClayworth above, that rules are not a way to solve the thing. Some of the ideas that spur in my mind:
- Face your players with situations, where the only way to survive and/or win is teamwork - the opposing team will steal their treasure or glory if the Defender and Tyrant don't work together;
- Put them in a long-term situation where they need each other - people generally hate the Tyrant and the only thing that stops them from whacking him up to bottom is the Defender, who wants to give him a chance. Perhaps this way he will change his ways? Or the Defender is in a prone position, completely reliant on Tyrants power.
- Rule-wise, you can award teamwork with: experience, better reputation, good luck.
- Alternatively you may punish those who are not too keen to work with their party members: experience penalties, bad luck, fatal consequences ("See? You didn't help me saving that peasant, now it's his spirit that makes all your food turn yellow!"), bad reputation.