When you're writing a story about a gay couple who can't express their love for each other, anything in the closet takes on a special significance. And that's just where we find Jack's two bloody shirts: in a hidden section of the closet. As symbolism goes, it's a trifle on the nose.
There's actually two shirts: one of Jack's with blood on it, and one of Ennis's that he thought he'd lost (of course, Jack had just stolen it as a memento). Jack put "the pair like two skins, one inside the other, two in one" (146). Pardon us while we reach for the tissues.
Since Jack's dad won't let Ennis have the ashes, these shirts are the only remaining link Ennis has to Brokeback Mountain. In that sense, they become kind of portable versions of Brokeback Mountain and all the forbidden emotions and enduring love that it represents. The shirts are intertwined, like Jack and Ennis. They're bloodied like Jack and Ennis. And they're part of the past, not the present: a past that Ennis can't return to now that Jack is gone.
And poor Ennis can't quit the shirts; "he drove a nail and on the nail he hung the wire hanger and the two old shirts suspended from it. He stepped back and looked at the ensemble through a few stinging tears." (156) They'll always remind him of what he had and what he can never have again, a nasty double-edged sword in fitted flannel.