XN-Matt We would never permit this in the fashion you described, since this could potentially be exploited down the line. It would be best for customer one to simply create a special user for customer 2 to login to customer 1 area. That way customer 1 could disable/remove that user at any point in time.
Allowing customer 2 to be granted system level permission to see customer 1 user-space is a huge no-no, and defeats isolation and such..
I would never want this to even be considered as a feature.
There is another option in any case, if this was a "thing" have customer 2 become a reseller who could then manage their downline customers, and move customer 1 over there.
I have a couple of smaller boutique agencies that do just this. This makes the reseller fully responsible for all their sub-tenants/customers. You do not even have to call it a "reseller" if you do not want to do so. In our case, we still have the "customers" in billing system and charge them for the value-add hosting we provide. They just happen to reside under a "reseller" account for the boutique web design firm for the purpose of design and updates.
Or create a system-level user that has access to only the sites you specify using the Collaborator user type from Users on the main Admin login. That is also easy to control.
