I don't understand why they expected this to be a double-cross, but if they've called it then I would say one of two things is taking place at your table. One, the players are predicting what's going to happen instead of allowing it to happen, which usually means they're unhappy with the pace of the story (it's too slow). Or two, the players are expecting something to happen and are not engaged by the idea of it anymore.
If either of these is true then "spicing up" the double-cross is a bad idea because a) it's just a twist on a tired twist and b) it's going to actually slow down the resolution, frustrating any impatient players.
What I would do is drop the double-cross, conclude the adventure without any twists, and pay attention to what they say about it afterward.