You need to find and download the "child" ROM to add it to the "parent" ROM.
The parent ROM is the 4-player World version called tmnt.zip. Find the 2-player variant, which is the Oceania 2-player file, called tmnt2po.zip. Extract both zip folders. Re-zip all the files together into a new zip, and name that tmnt2po.zip. Then add to Hakchi, and you'll have the 2-player version. You follow the same process with any other 4-player game that has a 2-player variant. I've done that with TMNT, The Simpsons, and Sunset Riders all successfully.
Be sure the name of the new ZIP folder is the same as the "child" ROM that is the 2-player version. If you keep the original name (as in tmnt.zip), MAME will not recognize the new files, and it will be 4 player.
Why would you be using the FBA core for TMNT? That runs on MAME2003.