When you log out and back in, Marketo automatically logs you into the subscription you were last logged into. You can then switch to a different subscription if necessary.
Single Sign-on (SSO) works with Universal ID. However, currently you cannot invite a user who wants to have access to second subscription that is SSO enabled. The user must opt-in first with a non-SSO subscription and then can be invited to a SSO enabled subscription. However, if a user is invited to a second subscription that's non SSO, they can opt-in and then can be invited to a third subscription that's SSO enabled.
If you have multiple subscriptions, you may have multiple community profiles. We recommend that you choose the login that's linked with your most active community profile.