I'm pretty sure QBO has a means of Sharing a company file with another bookkeeper and just allowing them access.  I realize this conversation is about 9 months old at this point but I am a bookkeeper and have two different sets of books shared with me by two difference companies and both appear under my login, and I choose which company I am going to work on any given day.  Both of them were existing companies and managed by others before I was given access.

One of them I am a user on the account  (QB allows for multiple users) and another I am an accountant that is given access (QB allows you to give access to your accountant.)