The only way I know how to do that would be to get a ceasefire with the enemy. Shower your proposed ally with gifts and then when you initiate hostilities against your enemy hope that your relationship with the proposed ally is better than between the ally and your enemy...

It'd be nice if the diplomacy system allowed you to spread propaganda about an enemy which could affect the interactions between different factions. Success would depend on existing relations and your diplomat/spies skill at propaganda.