The Tomorrow Joint Trust is a Revocable Living Trust created by two people instead of one. In this case, it's a Trust created by a married couple or registered domestic partners living in a community property state. The Joint Trust describes what will happen to your shared community property when either of you passes away.
The Tomorrow app automatically creates a Joint Trust when a person indicates in their profile that they live in a community property state. The Joint Trust is created when that person invites a partner or spouse and a corresponding account is created. Assets such as bank and investment accounts will be synced between both accounts inside the Joint Trust if a partner or spouse is added as a co-owner.
For more information about community property states, see this FAQ article.