I would also set the max server memory for each instance ensuring that in the event that both instances are running on the same node. You could have the "lower priority" instance max lower than the "higher priority" instance.

I have seen a thread at this site where someone is doing this with scripts on failover enabling them to dynamically change the memory. Since that is the case, you could also dynamically configure the cpu's via the "affinity mask" options if that is something you were looking to do.

Steve