Just like your first client, accepting the email to be an accountant user shoud take you to either your current QBOA account from which you manage all clients. My hunch is that the first client added you as a regular user and not an invited accountant and you have only one registered QBO account which happens to be not as an accountant user - or the second client is adding you as a regular user, not accountant - or both the above is true.
If any of the above you need the client(s) to cancel you as a regular user then turn around and invite you as accountant. You should not have to establish your free QBOA first but certainly as part of initial setup as first invited accountant.
