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Insight has its own installer just download it.

The insight license is always a separate thing from the RX/Ozone/Whatever advanced license.  You either have a license for it or you don't.  The "form" of the installer doesn't really matter.  The Advanced RX license only authorizes RX Advanced (which has different features from RX standard).  When you upgraded your RX you don't get a new Insight license, you just have one that came with the first Advanced product you bought.  I'm pretty sure I didn't even get more Insight licenses when I upgraded from RX3 Advanced to RX 4 Advanced to RX6 Advanced.  Nor did I get another one when I bought Ozone Advanced.  I have only one Insight license despite owning three advanced products (RX/Ozone/Neutron)
 
You can authorize more than one computer though as long as they are your personal computers.  Here's the details:
https://www.izotope.com/en/support/knowledge-base/installing-and-authorizing-on-multiple-computers.html
 
If you have two completely separate licenses (because you are a business perhaps) things would be different, but odds are you upgraded which means you don't have two licenses you just have one.  And you could put RX6 and Insight on both machines.

It looks like they aren't including Insight with Advanced products anymore.  Which may very well explain why I haven't gotten more licenses.  But my account does show/have a completely stand alone Insight license/serial number.  I'm sure iZotope's support can help you sort this out.