I mean I know the mechanic overall works as it should, perhaps there is an edge case or something? We will run some tests to verify that it is working as intended in the situation you described. If you test it again and see those results and can give us the real numbers (e.g. this is how much XP I got at this level, they got this much, this is what we attacked) that would also be helpful for tracking it down.
There are a number of factors that go into how much XP you actually earn (level differences are one, the bonus XP you get for secondary classes might be another) but in theory as you said the two kills should net the same amount of XP to each person between each attack…