D and S is Italian for right and left, leading to dexterity and sinister. Nobody likes a southpaw, apparently.
French pedal threads are slightly undersize compared to English threading, so strip out when mixed. L and R threaded as normal on the 3 sets I've owned.
You should be able to easily see whether they are opposite threaded.
Italian threads have a slightly different thread shape but should work. I'd guess that the thread tolerances are a little off in different directions for at least one of your pedals.