I would suggest the one who scores the hat-trick first... though these days with so many balls being used you probably won't score them all with the same ball so they'll probably just take one each.
It happened today at Wigan V Blackburn, Bent apparently didnt give Cruz the time to get the matchball!!
Would have thought it should be the first one to get the hat-trick. Then again - would he be that bothered about it when they lost the match? Are they that bothered about having the matchball anyway?