The other tank will always be in a fixed position on your party list, so I don't really think there's ever a need to have anything other than an /ac Shirk <2> (or whatever position you set them up to be in on your list). I can't think of a need to shirk anyone other than the other tank that doesn't involve trolling and laughter. The <tt> and <focus> works in theory but I don't think they are needed; <tt> has the potential to go wrong if someone else somehow takes threat (or is the target of an ability) at the time you cast it without you realising it, and focus locks up your focus target.