I am finding that, and this is more of a generalization of all fish as opposed to specifically about triggers, that basically, if they don't come in as an established pair, odds are not good that you can make a pair. To the point that I now get really annoyed at stores that will sell individuals out of a pair - if they come in as a pair, they should be sold as a pair, period. Because you can't really make pairs out of non-pairs.
I can only imagine how much worse it would be for triggers with their massive tempers.
I bet the only way you can do it is to have them in the same tank but partitioned off with eggcrate so they can see and smell each other but not inflict too much damage.
I tried recently to pair up two rabbitfish of the same species by introducing them into a new tank. I thought maybe because neither had seen the tank before that maybe they would, if not "pair up" per se, at least form a small school-of-two. Boy was I wrong. The smaller one was dead within 2 days. So my strategy of "wait 3 days to see if the aggression subsides" was the wrong call in that case.
And it's weird because I don't really see how the damage actually gets done. For the most part the aggressor seems to "swim really angrily" and the weaker one acts like it hurts. "Ahhhh! Please stop swimming so angrily!! <Gak!> <Dies>".
In short: fish are dumb, man. Dumb dumb dumb. Expect problems.
Having said all that I do find myself wishing I had a pair of crosshatches and keep wondering how I could ever take the one I have right now and add a second one and there be a successful transition into "pair-hood". Again though I think the only strategy would be to have a partitioned tank and let them see each other without hurting each other and then gradually remove the partition somehow over time (ie., remove it then replace it an hour later or something like that, then rinse and repeat). However there was a fellow on Canreef posting a couple weeks ago that he had done this same strategy with CBB's and it wasn't working out, they constantly were trying to fight each other through the eggcrate. So, I don't know.