"If there was a "double-cross", then why were these two hanging out in this area in the months following the canyon ambush? "
More to the point, why would they be off conducting what was likely a paid assassination of Mr. Peel if they had all that treasure buried somewhere? After the killing of Peel they went to Chandler's Milk Ranch and sent a messenger to Tombstone to retrieve money owed them. It was the messenger that alerted Breckenridge to the outlaws whereabouts.
Moments after the killing of Peel, witnesses stated that one or two (I forget how many) rockets were seen bursting in the night sky. I surmise they were signals that the killing had been conducted. No attempt was made to rob the mining office so I think it was an assassination.