By now you can configure multiple security systems. But a workflow application can only use one of these security systems as source for it's users / roles.
In theory there is a workaround: https://answers.axonivy.com/questions/583/how-to-read-and-synchronize-users-from-multiple-active-directory-domains
But I'd not recommend to take that effort. Especially as we are working in this area... so with the next leading edge releases the security system configuration becomes more flexible to covers cover use cases with 200k users.