Tested wall of flame + antivan fire on a mob. antivan fire does 89 dps, but the dmg number only increased by about 40 afterwards.
On the other hand wall of fire is dealing normal dmg so its not because fire resistance. Therefore it would seem that dot does stack, but further dot after the first one will do reduced dmg.
Note that this is just one test (I quickly ran out of grenades), so the conclusion is not necessarily accurate. The dmg number might not be popping up properly, or I could have just read it wrong. So don't quote me on that.