How do you trigger a double battle (with 2 enemy trainers) in eventing?

I want to trigger a double battle battle between you, your rival, your rival's father, and your rival's mother. So, I already know how to add the rival so that she will participate, but how can I make it so that you battle both the mother and father?

It's on the wiki:
http://pokemonessentials.wikia.com/wiki/Trainers#Against_two_separate_partnered_trainers

Correct-a-mundo. I've been trying to do that in Pokemon Pear Blue when battling 2 Nebula Grunts at once while partnered up with one of the Gym Leaders, which will release after a while....