How to load a 2nd copy of OneCast with a new license?

This is a clear violation of their licensing terms.

Our single-user licenses provide access to the software on one or more personally owned machines. Each user needs just one single-user license, regardless of how many machines the software will be used on. Please note that there is also a limit of how many different Xbox One consoles may be used with each license key, so don’t share your license key with others. Licenses found to have been shared will be restricted.

So, based on the behavior of the application you are describing, it is working as described - on a per user basis. You can install it on as many Macs as you like, but if the users are different, you're in violation.

I don't advise trying to circumvent this restriction as they seem to imply the ability to disable the software. If you need additional XBox connections or additional functionality, it's best to contact the developer.