Their names are Dupont and Dupond in the original version. You can distinguish them by their moustache:
- Dupont (Thomson) has a pointy moustache.
- Dupond (Thompson) has a moustache which looks like the letter D.
Note that there is also a character named 'Allan Thompson'. He was Captain Haddock's lieutenant, and often works for Rastapopoulos after that. He is a bad guy. Maybe his name is different in the English version to avoid confusion with Thomson and Thompson. Update: The Custodian gives the answer in his writeup above: Allan Thompson is only known as 'Allan' in the English versions.