Your two toons in an instance, with an 80 on another account boosting, will be the fastest.
If you don't have access to an 80, questing is as good on your own as instances, so do whatever you enjoy more.
But with the booster, definitely go instances.

Experience caps (per kill) with mobs of your level +4.
So if your toons are level 20, the most you'll get is the experience for a 24th level mob.
If your 80 friend can one shot a level 24 or a level 50 mob, its the same experience for your level 20's.

Ideally, go to the highest instance you can survive in, that your 80 friend can kill things (AoE is faster) quickly in.
That way, you can be there as long as possible before needing to move to another instance.

Even if your 20's are only getting the exp's of a 24th level mob, if your 80 is dropping say a level 30 in one hit, that's fine.
You can be there to 26th level, before somewhere else is better for you.
And wish boosting, it might be a great place to mid 30's anyway.
It's experience per hour that is most important, not experience per kill.