This will be of no benefit on search heads.
On an indexer, when you dispatch a search job, a process for each search gets created and run on each indexer instance that is in your cluster. So, if you have the CPU and IO capacity on each node (which you do), running multiple instances per node will allow each search to use more of that capacity on each node, and thus complete faster. (This wouldn't be the case if you have high search concurrency and either CPU or IO is closer to being maxed out. It will run more processes, but they'll each take longer.)
However, for any given search, you will only have a single search process on the search head. It won't run any faster or use more capacity with more search head instances. (It is in fact possible to run multiple full Splunk instances, including SplunkWeb, on a single node. But it won't buy you anything.)
Are you running on Windows OS?
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