Just figured it out myself - in the top-left menu, click your username and then +Add Account, then you can enter the URL of the Lemmy server you want to connect to along with your credentials. You can then switch back and forth between your logins on the different servers.
(edit) - Or do what @sexy_peach said below. I don't know what I'm doing. :P
Of note - while logged into any one instance, you can also pick from the various filtering options:
Subscribed - List posts only from communities you're subscribed to
Local - List posts only from communities in this Lemmy instance/server
All - List posts from all (federated) Lemmy instances/servers
So, selecting "All" at any given time will show you posts from all servers that are federated (share creds) with Lemmy.ml.