I'm pretty sure neither HB or WHMCS allow contacts to login to multiple different client accounts.
We were thinking ahead when we architected the authentication system, so it would be possible in a future release without too much work for a client to have access to multiple companies with the same login. The same may be true of contacts.. but it's not a feature that is widely requested, and nobody else does it to my knowledge. If someone else does do that, I'd love to check it out.