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 states what will happen to your shared community assets when either of you passes away.
The app is setup to start a Joint Trust when a person indicates in the profile that they live in a community property state. The Joint Trust is created when that person invites a spouse and a corresponding account is created. Each person should input children that will be named in their individual Wills. Assets such as bank and investment accounts will be synced between both accounts inside the Joint Trust.
For more information about community property states, see this FAQ article.