About 15-20 years ago I had 3 Delrin-ish plastic adaptors, that slipped onto the drawbar portions of the loco/tenders. One with a vertical slot, the other horizontal, and a small slotted set screw. Both non operating too. If you could handle a little more space between couplings, you could run a loco without a tender, using the drawbar add-on coupler. Or run two drawbar tenders in tandem with the other.
I used the "normal engine side" version, on my MARX for years, but haven't seen them around my train stuff for a few years now
Who made these! Who has more! They look similar to this MS-Paint job.