You can have a large number of ESXi Servers and a fair number of VMware Clusters under a single vCenter Server. I will admit, it has been a while since I installed View and whilst iirc, view composer -> vCenter is 1:1 does it need to be a dedicated vCenter Server for anything else you have? Are you not only doubling but splitting it into 2 View environments?
Assuming all hosts are identical configuration (and this should be a pre-req), I personally would create a new Cluster with the new host. Then put one of the 5 hosts into maintenance mode to evacuate the VMs, disconnect it and re-connect back to the vCenter Server in the new cluster (then you would have 4 in one and 2 in the new cluster) and then complete same exercise again to balance it out in 3-3.
Will the new cluster have different storage presentation or are they all going to see the same storage?
What is the perceived benefit of have two clusters in the first place? By reducing the amount of servers in the cluster to 3 you potentially reduce the capacity to fail. Two 3 node clusters (at lets assume n+1) could have less capacity than a 6 node (5+1) cluster depending on how you are doing HA (obviously that high level and it depends on how you configure HA)