Likely you would have to scavenge the part from a broken one. Occasionally you can find devices like this on Ebay or craig's list(be careful on this one) being sold for parts.
For replacing the sensor screen you'll need to open up the device, you can follow the steps in this video until opening it up. Once it's open you'll need to poke around and remove the sensor from the interior/bottom of the watch. On the other hand if you do get a defective watch (such as one with a broken screen) you can instead remove the bottoms from both watches and swap them. Any tools you'll need can be bought here on ifixit, but all you should need is an antistatic wristband, a pair of tweezers, a small phillips head screwdriver and a very small flathead screwdriver (Or any thin flat tool you can use to pop apart the side, be careful because if you use a tool that's too big and deforms the plastic you'll lose some water/dust resistance.)
I hope this helps!
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