Powerleveling a second Battle Pet after leveling one up

[MetalGenocide]

So what I did earlier today was start fight with high level pet > get mobs health down > swap to low level pet > deal killing blow with low level pet.
Is that how you described it ? If so I found my XP to be shared.

What level were the 3 pets used?
As I understand, your high level pet needs to be 9-10 levels higher then the one you are fighting (enemy pet is grey) in order for the low one to get the full exp.
Also, the technique used by the OP is to use a delayed ability such as Lost of Lordereons Curse of Doom (level 20) then switch to the low level that should be leveled and let curse of doom kill the enemy.