Pyromancer's Assault

Whenever you cast your second spell each turn, Pyromancer's Assault deals 2 damage to any target.

Pioneer Masters

Never ask a pyromancer if that's the best she's got.

Oath of the Gatewatch

Whenever you cast your second spell each turn, Pyromancer's Assault deals 2 damage to target creature or player.

The ability can trigger only once each turn. The ability will resolve before the second spell resolves. It doesn’t matter if the first spell you cast that turn has resolved, was countered, or is still on the stack.