I had seen the "Additional Programs" field used in Epics before, and also noticed it is mentioned in Help. Help seems to imply that you can enable this field for specific Portfolios. I couldn't find this anywhere in Administration -> (Settings) Platform
How do you enable this field?
Once that is turned on, you'll see a '+' sign to the right of the Programs option in your Epic.
Click the '+' and you'll get a new field for adding other programs.
One thing to remember: if the different programs have different PI cadences, you need to include both PIs in the Epic's Program Increment field. In the example above, if the Mobile program and the Align Implementation program have different PIs, they both have to be included.