please add the link of the document you mentioned on your post :
The example you are giving does not qualify as multiservice:
in your example it seems you want to make one collector to collect all devices. which you would only need to add those devices to your collector if they were reachable from your collector - and then they would need to be onboarded the users of the other and they Portal instance would need to be decommissioned
If you cannot access the devices they have ( access list , firewall, routes and so on. ) you could simply create another certificate with a different inventory name on your portal instance and they can replace the certificate on their CSPC , this way that collector now would belong to your portal instance and you can see them distinctively by inventory name or combined by all . It seems this one would be more appropriately. The users on their portal still would need to be migrated to your account and that portal be decommisioned
