We are willing to understand the approach and the licensing requirements in order to install Splunk ES on clustering on Indexers and Search Head. Will we need an identical license on both the clusters?
ES architecture and dimensioning isn't an easy job, I hint to engage a Splunk PS or at least a Splunk Architect with experience in ES architectures, because there are requirements and attention points different than Splunk Enterprise, (only for example: for Splunk Enterprise you use one Indexer to index until 200 GB, with ES until 100-150 GB.
Hardware requirements are described at https://docs.splunk.com/Documentation/ES/7.0.1/Install/DeploymentPlanning but as I said, it's mandatory to have training and experience on ES.
Ask to your reference Splunk Partner to help you; if you are a Splunk Partner ask to your managers to partecipate to a training (I did it!)
if you have one cluster, that make data replication between the peers, you have to pay only one license (if you have a license for indexed logs).
It's different if you have two (or more) clusters that exchange logs between them: in each case you have to pay the total indexed logs, not also the replicated ones.
It's obviously different if you have a license for CPU.
My hint is to ask to your reference Splunk Partner or a Splunk Sales.