I know this is an old thread but thought I'd post this in case somebody else is looking for answers as I was a few days ago. A DOS-compatible cable with two optical sensors is being sold on eBay. Just got mine and it appears to work as expected. I plugged it into the DOS and Apex shows a DDR and both sensors attached.
https://www.ebay.com/itm/352275690018
https://www.ebay.com/itm/352275690018