I'm positive you will be fine in doing this. Its an NFS datastore and your storing files on the data store.
Each vmdk will get its on directory on the ifs mount and each ESX server will only care about that vmdk which is started with its resources.
So the fact that the two clusters are of different ESX version does not matter to the NFS share. The only thing I would caution would be to ensure the vmdka file compatibility between the two cluster if you ever needed to failover a sever to the higher or lower version cluster.
Bottom line is you should be more than ok sharing an NFS mount between two ESX clusters.