thank you for your answer.
So this means:
We can run a high-available Portal (two Portals with loadbalancer) as host which will receive updates from high-available Portal (two Portals with loadbalancer) as guest?
The communication is bidirectional:
when uploading the invite-file and also when uploading the response-file, in both cases, the https-certificate of the other server is checked.
Or can i tweak this, that it will not be checked?
Establishing trust is not the same as bidirectional communication. The invitation cannot be accepted if the SSL certificate of the host isn't trusted; if you're getting an error then you'll have to import the root/intermediate certificates into your Portal - Configuring the portal to trust certificates from your certifying authority—Portal for ArcGIS | Docu...