I’ve asked about multiple partner accounts but I think the answer is still no for know. I’ve been creating dev admin accounts per site and then I have extra FTP credentials that I have just for external devs. The ftp and assignment to sites is done through a support request.
As @Alex_B_Centrifuge mentions, it is not possible at the moment, but you are welcome to add it to the backlog.
With BC if you logged in with the same user, you would kick each other off the site. My experience with Treepl is that multiple users can be logged in with the same admin credentials and work on the site simultaneously