Are both of your fires on 0 SPD?
Becouse, if not, rememer they must be slower then the pets, and if one fire is faster then the other (yet still slower then the pets you burst with),
try for the faster fire to:
Slow - poision - GS - attack
and for the slower one to:
Poision - slow - GS - attack.
That's how you normaly burst.. yet I advice you to make a full team, it might not work becouse it only lands for ONE fire and becouse the other also attacks noone gets the exp.
Also combo = both fires to attack at a time, if they won't combo try to make them get 0 SPD.