The Enroll in Another Journey flow node allows you to enroll a contact in another Journey. See Fig 1 below.

Fig 1

To select which secondary Journey the contact will be enrolled in, click the Enroll in Another Journey flow node once you have added it to the Journey work area. Doing so will bring up this popup ("Properties") on the right-hand side of the screen. See Fig 2 below.

Fig 2

Clicking into the "Journey" input field below the "Journey" label will allow you to select another Journey in which to add the contact. Please note: Only ACTIVE journeys targeting the same database will be available as options.  See Fig 3 below.

Fig 3

Once you have finished with the action's setup, click the "Save Properties" button.