Hi @mGéza_,
I don’t believe there is an official “solution”. It’s just how it works, unfortunately.
The respective email address would have to be a user in both systems if you want to log in with one email address and access both HubSpot portals.
This is a significant workaround, but you could log into your own HubSpot account with your customer-provided email, but you would have to add that email as a user in your own account. In this case, the same user would exist across multiple accounts, allowing you to log in one time.