Can you implement the exact same SS-GA4 tracking across two separate top-domains?

The customer uses a third party tool for some pages/forms, which are running on an external domain. It is impossible to set up a custom subdomain for JENTIS within the same domain. In this case, would it still be possible to run JENTIS and a server-side GA4 implementation without any problems?

In addition to having an “isolated” GA4 property for this domain, do you also support cross domain tracking now?

The answer: 

The answer is that No, unfortunately we still don´t offer classic / automated cross-domain tracking due to legal and tactical conflicts about how to execute that. 

Meaning the answer to your specific question is also no- it will not be possible to run JENTIS and a serverside GA4 implementation without any problems on the external domain (2nd top-level domain). The only workarounds available: 

• set up a different container for the ext domain and cross-reference data, potentially with timestamps. 

• use link decoration as per the article Cross Domain Tracking here to execute it.