Spelltwine

The copy you cast first will already be on the stack when you cast the other copy. If that spell targets a spell, you may choose the first copy as a legal target.

If you cast a card “without paying its mana cost,” you can’t pay any alternative costs. You can pay additional costs such as kicker costs. If the card has mandatory additional costs, you must pay those.

If one of Spelltwine’s targets is illegal when Spelltwine tries to resolve, you’ll still exile and copy the remaining legal target.