http://prntscr.com/l07md1
There is a default trainer on Route 3 that battles using two Pokemon at the same time.
The other way to do it, if you don't want to use comments, is by turning one of the "false" parameters into "true".
pbTrainerBattle(PBTrainers::LASS,"Crissy",_I("You didn't have to win so convincingly!"),true,0,false,0)
The true means that it's a double battle.