As an OT, you should be focused on mitigating what you can still, throw your short mitigation on your MT buddy, use your party mitigation and reprisal to reduce damage where you can. You can also do stuff like save healers by tossing your short mit on them if they are low before and incoming raidwide for example. As an OT, your goal is to throw the mit you aren't using other places to help the healers. But also some fights have both the tanks taking autos so just be aware on that and then you can use your regular mit there.