For what it's worth, Petzl provides drop-test data on the Tibloc. For an 80kg mass (your typical climber, I suppose) taking a factor 1, the device completely cuts 8mm cord, but no failure on high stretch 9mm or greater. It is marketed for 8-11mm ropes
Slow pull tests on the Tibloc (to sheath failure):
8mm - 4.9kN
9mm - 6.5kN
11mm - 5.3kN
Falls generating forces of this magnitude in simul-climbing are conceivable if there is slack. This data doesn't say when the rope will snap, but will tell you when it is well on its way to failure.