Buttondown makes it easy to manage multiple types or segments of subscribers using tags, but sometimes that's not enough. If you want to be able to completely separate out multiple verticals of a newsletter, keep track of different archives, or have two entirely different publications with different subscriber bases, your best bet is to set up separate newsletters entirely.
To add another newsletter to your account:
After you add a newsletter to your account, you don't need a new login or username or anything. Just open the same dropdown you used to add a newsletter. All the newsletters in your account are displayed there. Click any different newsletter to switch to it.