Chris,
On page 55 of Bary's book.
"Each TIU used with a layout must have a unique TIU #1 through 5. Numbering TIUs is required by DCS.
Numbering of TIUs is accomplished through the DCS System menu."
I would start there. If that does not work, you may need to do firmware on both TIUs.
Hope this helps.
Regards,
Robert