Figured out an acceptable temporary workaround. My initial Bubble (index) page checks if the user is logged in or not; if not, it 'opens external website' and redirects to my landing page, if so, it proceeds to the internal site 'first page'. On the external site there's a 'login/signup' button that then links to my login/signup Bubble page. I wanted to use Bubble's built-in functionality for simplicity, I think I could have used an API from the external site if I so chose.
My domain names aren't exactly the same, but close enough for this stage in the game for me. Once I get full functionality I'll come back around to cleaning all this up.
Hope this helps!