Hi Łukasz,
In my opinion, what you are describing is possible if you have the appropriate WEBCON licenses.
In my opinion, combining two WEBCON installations into one is possible in the following way:
1. Create new CONTENT and ATTACHMENTS databases – the same number as in the target environment
2. Restore the databases from backup
3. Adjust the process configuration to the current installation / database connections / integrations, etc.
4. Reconfigure application/process/global GUIDs, etc. so that they do not duplicate other databases
5. Synchronize users and update them with the new domain if necessary
There are certainly more points, but what you need is analogous to the mechanism of dividing a single content instance in the current environment into multiple instances and distributing processes/applications to individual new databases.