If characters are the SAME level they will stay the same forever due to the class/race modifiers giving the person with the penalty more XP per mob than the other, thus keeping it even. So someone with a high penalty gets more XP per kill than a lower penalty and you will ding at the same time, because the higher penalized person gets more per kill to compensate for their penalty.
If they are different levels -- the person with "more XP" gets more of the XP per mob kill and will eventually get further ahead over time. "more XP" is represented by the total amount of XP they currently sit at.
For Example a Human Bard just starting level Level 31 has a total currently earned XP of
41,580,000.
If they group with a 29 Human Druid who has just dinged 29 and has a total earned XP of
21,952,000.
Add the two numbers together and take each players current XP level and divide by the total. That will give you how much percent of the the XP each kill offers each player will take. In the 31 bard and 29 druid example. The Bard takes 65% of each kill and the druid takes 35%. While that seems like a huge problem, it's not THAT bad if you kill fast enough.
Using the same example. If the 31 bard and 29 druid are killing level 27 mobs in a zone with no presumed XP bonus, it should take the Druid no more than 8-10% more kills to complete his current level than the bard did (31-32 and 29-30 respectively)
I made a calculator to test these things theoricially back when Green was in Beta just for fun. It's obviously not perfect because ZEMs aren't perfect but I have tested it with my duo and in alot of zones the numbers are usually within 5-15 kills, so again, not perfect but the XP split part should make sense because that part is just math.
Link is here:
https://docs.google.com/spreadsheets...it?usp=sharing
Save a copy to your own google drive if you want to play with it. (directions on right)