Hi,
It is possible with 1 vCenter only. No need to have 2 different vCenter servers for 2 different cluster.
I did the same thing. I had 2 clusters running esxi 4.1 hosts being managed by 1 vcenter 4.1. I upgraded vCenter to 5.1 and then ist cluster to esxi 5.1. So i i too was managing 2 different clusters 5.1 and 4.1 with single vcenter 5.1.
It is backward compatible.
Thanks,
AG