Can I use the Illusionist's Bracers and the Twinned Spell metamagic to cast a twinned Booming Blade as both an action and a bonus action?

Question

The description of the illusionist's bracers (GGR, page 178) says:

While wearing the bracers, whenever you cast a cantrip, you can use a bonus action on the same turn to cast that cantrip a second time.

The sorcerer's Twinned Spell metamagic option (PHB, page 102) says:

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).

Can I use Twinned Spell when I cast booming blade (SCAG, page 142) as an action, and using the illusionist's bracers, twin-cast booming blade again as a bonus action (thus attacking 4 targets or the same 2 twice each)?

Answer 1

Update: the Sword Coast Adventurer's Guide errata from November 2020 changes Booming Blade's range to self, making it uneligible for twinning specifically. The following applies to Booming Blade only if you don't use that errata.

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The only general limitation on metamagic, as stated in PHB, is:

You can use only one Metamagic option on a spell when you cast it, unless otherwise noted.

But there is no limitation on how many times you can use metamagic per turn. And the castings with these bracers are explicitly written to be two castings (first as an action, second as a bonus action), as quoted in your question.

So yes, you can twin the spellcastings as you describe, an action and a bonus action, in the same turn. You do have to spend sorcery points for both instances of Twinned Spell (2 sorcery points total, as both spells you're twinning are cantrips).

Answer 2

Yes, you can use two castings of twinned booming blade

Booming blade is a cantrip that targets only one creature, and as such it is eligible for the Twinned Spell Metamagic option. As long as there are two targets within the 5ft range of the spell you can twin the spell to attack both of them. Illusionist Bracers allow you use a bonus action to cast the same cantrip a second time, so booming blade can be cast (and twinned) again as a bonus action.

Note that the booming blade effect will not stack on the same creature more than once, due to the rule on combining magical effects:

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect - such as the highest bonus - from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

If the second casting is on the same two creatures as the first, they will still only be affected by one instance of thunder damage should they choose to move.