About co-tanking

[Terkhev]

As PLD you never pull - DRK and especially WAR have much better snap aggro and lose far less DPS by starting in tank stance. There are exceptions: other tank has much worse gear, can't tank, fell asleep or just refuses to pull. In pre-70 content it doesn't matter that much.
Another important thing is OT/MT misconception: in savage content tanks should swap all the time to fully utilize their cooldowns. There is no main and offtank, unless you just use those names to describe which tanks has aggro at the moment. Again, doesn't matter pre-70. Nobody is going to bother with swapping during normal trials.

So yeah, as PLD you should just be blue icon DPS most of the time :P

One more note: around lvl 50 is really weird for tanks because of their DPS rotation. WAR doesn't have his stance and PLD doesn't have combo. So even if you don't tank you will get a lot of enmity - Shirk helps a little.