In reply to petegunn:
Further to the points made above, if you are using this in guide mode then there should be very little shock load in the system, so not much more than body weight (x2) on the device.
call it 250kg so about 2.5kn
from memory the load for a lead fall factor 1 on dynamic rope is ~ 8kn so if you somehow manage to get both climbers to lead fall whilst using guide mode, which would be quite a bizarre situation, it would still be below the rated strength of the device although the anchor would take a thrashing!