Exile target instant or sorcery card from your graveyard and target instant or sorcery card from an opponent's graveyard. Copy those cards. Cast the copies if able without paying their mana costs. Exile Spelltwine.
Spelltwine has two targets: the instant or sorcery card in your graveyard and the one in an opponent’s graveyard. You can’t cast Spelltwine unless you can choose both legal targets.
If one of Spelltwine’s targets is illegal when Spelltwine tries to resolve, you’ll still exile and copy the remaining legal target.
The copies are created in exile and cast from exile. The cards remain exiled.
You can cast the copies in either order.
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 the card has X in its mana cost, you must choose 0 as its value.