I set up a Thunderbird account for my husband. How can I set up a separate account for myself without affecting the existing account?

I set up a Thunderbird account for my husband, using his Mediacom email address and password. How do I set up a second Thunderbird account using MY Mediacom email address and password? If I use the "Add Account" option, it screws up access to his account. Adding another "identity" doesn't seem to work either. I remember having trouble with this many years ago when we first began using Thunderbird, but I don't remember the"fix!"