I see two main issues with the separate tanks.

First is that with a shield PLD has a consistent damage mitigating ability that WAR does not. WAR have 25% more hp but only get a 15% boost to heals while Infuriated. Thus the issue you mentioned with healer MP in longer fights, or even just damage heavy fights.

Solution: Make Inner Beast apply a Damage reduction buff of say 10%, you lose the +healing but gain a little mitigation.

Second is that PLD have a large number of "Oh S**t!" buttons while WAR have ones of limited use.

Solution: Either increase Bloodbath healing received via the methods mentioned above or make Rampart cross-class. Get rid of awareness for rampart, not useful enough as our higher health pools make crits less dangerous.

Another solution is to roll the current Foresight into Defiance and change Foresight to something useful.