You can copy a player from one team to another so they are active on both teams simultaneously
Copy Player
Navigate to Teams in the menu bar and click into the team the player is currently on
Check off the member (or multiple)
Click Actions > Copy
Select the Team you'd like to move them to & Click Copy
They will now be on both teams and be able to see both teams within the app
Note: This creates separate player records, so the statistics for the player will not be combined as they are considered separate players