Don't think Tatum needs a nickname tbh. JT or Tatum are unique enough.
Jaylen could use one though. There are a hundred Jaylens, Jalens, and Jaylins in the league and a few Browns as well. He's not even the only JB of note (both Jimmy Butler and Jalen Brunson share the initials). Drew Carter pushes the Gravedigger occasionally, and... eh.