Can the sorcerer's Twinned Spell metamagic and the Enchantment wizard's Split Enchantment feature be used at the same time?

Question

The Twinned Spell option for the sorcerer's Metamagic feature says (PHB, p. 102):

When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).

To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level. For example, magic missile and scorching ray aren’t eligible, but ray of frost and chromatic orb are.

Similarly, the School of Enchantment wizard's Split Enchantment feature says (PHB, p. 117):

Starting at 10th level, when you cast an enchantment spell of 1st level or higher that targets only one creature, you can have it target a second creature.

If a character with access to both Twinned Spell and Split Enchantment casts a spell that fulfills the requirements for both, can both features be used at the same time?

Answer 1

Each description specifies targeting a second creature.

Each feature description is specific to state that we may “target a second creature”. Twinned spell says:

target a second creature in range with the same spell.

Split enchantment says:

you can have it target a second creature.

The first three ordinal numbers in English are first, second, and third; not first, second, and second. So if we target a third creature, we are contradicting both class feature descriptions which specify we may only target a second creature.

We may only target up to two creatures, even if using both of these features.

But we can’t use them together anyway.

Twinned spell says:

When you cast a spell that targets only one creature

Split enchantment says:

when you cast an enchantment spell of 1st level or higher that targets only one creature

Each feature modifies the spell to target a second creature, so once we use one, our spell is no longer a valid for the other.

While Jeremy Crawford’s tweets are not official in any way, it may still be helpful to observe that he personally affirmed this particular ruling when he tweeted:

Split Enchantment and Twinned Spell are mutually exclusive. When you use one of them, the spell no longer targets only one creature.

Xanathar's Guide confirms this ruling.

Xanathar's Guide gives an optional rule for adjudicating simultaneous effects:

If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen.

Under this rule, the effects are resolved in order, not at the same time. Without this rule, see the first section - you cannot target three creatures.

Answer 2

RAW: Yes

If a spell has a single target, then even after the effects of Twinned Spell or Split Enchantment, the spell itself has a single target. Neither feature modifies the spell.