Ok, as usual, please excuse me for my bad english.
Some quests require you to have a specific partner out to complete a step of the quest. Sometimes, you then have to fight some mobs while your partner does not help you because he is doing something else (like opening a door of a temple or watherver).
Here is what you can do:
1) Summon this specifc pet
2) launch the step of the quest (like click on the door on my example)
3) At the end of the cast of the step of the quest (like opening the door), summon another pet.
Here you can see in the background the specific partner required who open the door while I have been able to summon my other partner Khem Val to fight with me the waves of opponents who will attack the first partner.
P.S. Feel free to rephrase this exploit if you understand it and speak better english than me.