☒ It would depend if they should be two users or a single user within the system and how switching tenants would work. That's more of an architectural decision that has to be made. If they should be two separate users with the same email, you would need to include some sort of indicator for which tenant they are signing in to (i.e. subdomain). 

If they will be a single user, then you will need a table with the user_ids and the tenant_ids. I'd also have something that would trigger their session being tied to a single tenant. So, they would have a current_tenant which has one tenant selected from their has_many :user_tenants (or something similar).