In reply to mcawle:
1. with a 9.8 and a 7.8mm large slippage can occur, as I've seen it! With closer diameters, not sure. It is more a creep in one direction, than sawing. (Going off question, this creep can be used with similar rope diameters, to useful effect, if you have enough rope ,as the last person down can make sure the knot is over the edge and away from cracks.)
2. If both people put their devices on the rope at the same time (on teether) the rope is unlikely to creep for the first person down. This means both ends get to the next station, however, the second can be exposed to creep.
3. Place the knot on the side of the thinner rope if possible. If not, due to multiple abseils, always use point 1 above.
4. Probably best just to sacrifice a krab if no rap ring.
5. In my experience, if no ring/krab then pulling the rope can be quite hard, using a pull line can then be really tough if there is an edge below the upper station, or other sources of friction