... where I use XP, it's all shared.
It's also not clear to me what kind of behavior you're trying to incentivize with this method.
What is shared XP? Each encounter is worth N XP, and that amount gets divided evenly amongst PCs?
The effect I'm going after is that PCs get XP for accomplishing class goals, but because other party members can earn you XP for accomplishing your class goals, you have an incentive to help them be better at it.
That's an awful lot of adjudication and judgement-calling for the purpose of deciding who gets to level up. It sounds custom-made to cause arguments at the table, and even when employed with complete fairness and transparency, its effect will be to create level disparities based upon the group's current activities. E.g. if their current mission involves a lot of investigation and infiltration, and not much actual combat, the rogue is going to be steaming ahead of the fighter.
You're right about the judgment calls, so a sturdy framework would be needed for determining what is and what isn't worth a point. But now you have me thinking that PCs could distribute their own XP to others...
And how do you prevent it from becoming a vicious circle? The lowest-level character in the group is inevitably going to be the one least able to contribute effectively, so the group will naturally gravitate away from relying upon that character's skillset, and when forced to do so they will achieve lower levels of success and thus earn fewer XPs. Meanwhile the highest-level character will have a breeze dealing with problems matching his skillset, and may in fact be the MVP even when tackling problems he's less than perfectly suited to, simply due to having a higher general competency level. The natural tendency will be for level gaps to widen as the campaign continues.
The XP-from-comrades feature might mitigate the problems that the lowest-level character might have. If you (somehow) suck at the one thing you're supposed to do well, then you have even more reason to get the rest of the party involved, helping you out.
Sure, the MVP could be better, mathematically, than the FNG. But it's not a problem if they have different class goals.
For example, Mister-Victory-Person is the party paladin. Paladins earn XP for protecting the weak. Fairy-Nighting-Gale is the party bard, who earns XP for entertaining/drawing attention. Since MVP is good at everything, he can save the orphans by drawing the dragon's attention away from them, and possibly get better dice-results than FNG. But this earns him primary XPs for saving the orphans, and zero XPs for drawing the dragon's attention, since it's not his class goal. FNG gets secondary XPs for MVP's attention-drawing based on the idea (or fact) that he suggested the diversion to MVP, lent direct help to MVP to make him more distracting, or in some other way helped him to get the dragon's attention.
So even if the FNG can't survive a dragon diversion, because he's the lowest level character, he still gets XP if the hardier party members pursue his class goal. Which...helps him to catch up?