Eating feces is a normal behavior for a dog. As disgusting as it is to us, it's normal to a dog.
Wild canids consume feces as a part of their diet. They especially prize that of herbivores. Ever see your dog around sheep, goat, cattle or horse manure?? You can almost see them go MMmmmmmm....Yu
mmmm as they eat it. LOL
Consumption of the feces of other carnivores is not as common, but it does happen.
Here is what I would do:
1) Observe the elimination of both your dogs, and remove all waste promptly.
2) if you have a HELPER, take the dogs out, and the OEM on leash. Take him near some of the other dog's waste. When he looks or sniffs in that direction, correct him verbally, "AHNT AHNT, LEAVE IT!", and tug him towards you (and away from the no no) on the leash. If he knows how to sit on command, ask for a sit, praise and food reward him when he does. I would let him focus on the no no several times and correct him the same way. You are getting your point across when he is paying attention to YOU, and won't look at the poop.
Practice this once a day for 7 days running.
When he cannot be distracted by the poop, then I would let them out off leash to do their business. I would still be there to pick it up right away, but this should prevent him from grabbing it before YOU can.