Had a good game today, my Necrons vs Sisters of Battle. The following situation came up;
My Wraiths inflicted 12 wounds on a depleted unit of 3 battle sisters (2 troopers and a leader).
4 of these wounds were rending hits.
Since the three sisters were only 1 wound models, there was no multi-wound nonsense to worry about. However, since each model would be taking 4 wounds, my opponent asked if he could assign all 4 rending hits to one model, then 4 each of the remaining 8 normal wounds to the other two models. I briefly checked the rules, but I couldn't find anything to suggest that the rending hits had to be spread out among the three models.
I allowed it, since it was only a friendly game and my opponent is a nice guy. As it turned out, all three sisters died anyway!...