Just leave the house and let the timer go off...
Spoilers...
It should have been evident to them once they made the cameras face each other, the extent of the mirrors power in controlling them.
That should have made them deduce that their plan was not going to work as long as they were in the circle of influence, as the mirror could easily from that time onwards do what it did at the end.
So why didnt they just leave the building at that time and wait for the timer to go off. Thats all they needed to do as it was clear they were still themselves.
I mean the mirror even showed them how it would protect itself by making them stand infront of it, and they still couldnt figure it out.
Were it not for that the movie had potential.