Agreed, but treat the sites like sub-organizations, set one site as Main or Primary and then add branch offices or whatever you want to call them. Do it under the main site then all the Companies data and details are under one main Company, but separated into offices with office specific passwords, docs, users, assets etc. Assign the client access and they get access to the main site/org and all sub-orgs. Give the option to exclude permission for certain sub-orgs for role separation and security.

Seen this in request in the past just FYI.. the client portal has been neglected for too long overwall!