I guess the big question is do both instances run at 60% utilization every single second of every single day 24X7. My guess is no. If it does then you will see some slowness yes. If not, then most likely you won't see any problems.

On a SQL 2000 cluster I had an issue with the CPUs being pegged at 100% for hours on end and it would cause the cluster to fail over. This is because the heartbeat check would time out and it would think the other node is dead so it would fail the cluster over to the inactive node. We did increase the timeout which helped a lot. However, we fixed some bad code and also installed two more CPUs in the one node and licensed it that way.