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:

1. 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;
2. 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.
3. Rule-wise, you can award teamwork with: experience, better reputation, good luck.
4. 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.

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:

1. 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;
2. 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.
3. Rule-wise, you can award teamwork with: experience, better reputation, good luck.
4. 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.

Extension

As per OPs follow up, a couple of suggestions:

1. All characters need need to come from one family/organization/agency/religion/philosophy
2. Each character needs to share goal with at least one other
3. Each character's back story has to be connected with back story of at least one other character; ideally they were friends or good buddies
4. Characters get additional 25% of experience for encounters or situation where they employ nice teamwork
5. Employ hazardous (but not lethal) bad luck for characters which don't work with others when they get a chance