How_Experience_Works
Based on my understanding of this article, experience should be divided based on how much experience each player has earned. In this way, grouping with a class with an experience penalty is exactly the same as grouping with someone of a higher level.
So, given that two level 10.00's are grouping together, one is a paladin (40%) and one is a cleric (0%); the paladin has already accrued 40% more experience. Therefore, he will take a 40% larger share of the experience. In this way, those two players will stay the same level basically ad infinitum.
However, in the case of two players starting out at level 1, both players will have equal experience. If they continue to group together, in the case above, the cleric will pull ahead by a few levels. Of course, the quadratic nature of experience requirement growth means that the cleric's leveling will slow down to match that of the Paladins, after they have gained a level or two advantage. They will still each earn an equal share of experience, because they have an equal amount of experience.
You can quickly solve this issue by un-grouping and getting the penaltied class to an equal point in level/%xp, at which point he'll have 40% (or whatever) more experience. At that point, grouping together should mean leveling at the same speed.