Hearthborn Battler

Haste
Whenever a player casts their second spell each turn, Hearthborn Battler deals 2 damage to target opponent.

Malcolm, the Eyes

Flying, haste
Whenever you cast your second spell each turn, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Mad Ratter

Whenever you draw your second card each turn, create two 1/1 black Rat creature tokens.

Improbable Alliance

Whenever you draw your second card each turn, create a 1/1 blue Faerie creature token with flying.
{4}{U}{R}: Draw a card, then discard a card.

Irencrag Pyromancer

Whenever you draw your second card each turn, Irencrag Pyromancer deals 3 damage to any target.

Jori En, Ruin Diver

Whenever you cast your second spell each turn, draw a card.

Breeches, the Blastmaker

Menace
Whenever you cast your second spell each turn, you may sacrifice an artifact. If you do, flip a coin. When you win the flip, copy that spell. You may choose new targets for the copy. When you lose the flip, Breeches, the Blastmaker deals damage equal to that spell's mana value to any target.

Ian Malcolm, Chaotician

Whenever a player draws their second card each turn, that player exiles the top card of their library.
During each player's turn, that player may cast a spell from among the cards they don't own exiled with Ian Malcolm, Chaotician, and mana of any type can be spent to cast it.