Siren Investigate was originally a Kibana 5.6.9, which Siren extended with many elements of Kibana 6/7 and others.  

If we consider the "stock elasticsearch" (open source) Siren Investigate very considerably extends what's possible in Kibana. Please see our page on this on the portal.

There are however reason why one might want to run Siren Investigate and Kibana at the same time, typically to use features, that Elastic adds to their "subscription" (not Apache Open Source licences) . 

There is no problem in doing this, just make sure that the configuration objects are kept in separate indexes.