First, I believe it is the case however you are able to kill enemies much more quickly with 2 people so it might outweigh the reduced XP.

Second, what the above poster stated is correct and level difference will reduce how much xp both parties get.

It is worth mentioning that XP from grinding enemy kills in the open world will very quickly become too low to matter for leveling purposes so it's not as much of a detriment as your level increases and the added kill speed to complete quests or whatever is much more useful than the extra XP you would get from individual kills while solo. If you hop into a dungeon, though, you can gain incredible amounts of XP from the kills within.