Two clusters with 5 nodes each or 1 cluster with 10 nodes

Hi,
We have two indices (index1 and index2) each with 10 shards and 1 replica,
on a 5-node ES cluster. One of the indices (index2) is continuously under
heavy update operations and thus hurts the search performance on the other
index (index1).
Typical search completion time on index1, before adding index2, was under
600 ms. Now it takes more than 2 seconds. So, we are thinking of adding 5
more nodes to the cluster.
Should we make 2 different clusters (of 5 node each) hosting each index
separately or just add 5 nodes to the same cluster (making it a 10 node
cluster)? Which one would be more performant?

Cheers
Nitish