So normally you can't twin shadowblade because of range self, but if used in conjunction with booming blade it's now possable to twinned spell it. As a DM would you allow this.
Now twinned spell wording says your spell targets a second target with in range, booming blade says you make a weapon attack as part of casting the spell so would a single weapon attack be compared to both armor classes of the two targets. If yes would you also be able to use sneak attack/smite dice against both targets seeing it would be a single attack role?
Finally if you used quicken spell booming blade twinned spell could you do all the nonsense twice in 1 round?
Booming Blade requires a melee weapon worth at least 1sp but the Shadow Blade is a spell effect and has no inherent monetary value.
You cannot both quicken and twin a single spell. You can use only one Metamagic per spell, except for empowered and seeking which can be stacked with others.
Being able to hit four times a turn with a Booming Shadow Blade is quite a lot of nonsense. :-)
Gruntler is right but there are also a few other things you are ignoring if you allow this:
Sneak Attack can only be applied once per turn and only to one target. Meaning it can only be applied on a single enemy, on one attack if the conditions are met, that's it.
Reference in the first paragraph of the Sneak Attack feature (emphasis mine):
Beginning at 1st level, you know how to strike subtly and exploit a foe’s distraction. Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.
Notably, it says "per turn" and not only during your turn so you can still use it if you get an Attack of Opportunity as a Reaction on somebody else's turn but that's the only way to apply it more then once.
Even if you could still Twin Booming Blade (you could before Tasha's changed it and Green-Flame Blade's range to Self specifically to prevent this) or if you Twin any other spell, each target is treated seperately. That means two attack rolls, rolling damage twice, etc. The question was already answered by Mike Mearls:
For the record, the only metamagic options you can use with another are the Empowered Spell and the new Seeking Spell one which both specify that they can be used with other Metamagic options but those are the only ones.
So normally you can't twin shadowblade because of range self, but if used in conjunction with booming blade it's now possable to twinned spell it. As a DM would you allow this.
Now twinned spell wording says your spell targets a second target with in range, booming blade says you make a weapon attack as part of casting the spell so would a single weapon attack be compared to both armor classes of the two targets. If yes would you also be able to use sneak attack/smite dice against both targets seeing it would be a single attack role?
Finally if you used quicken spell booming blade twinned spell could you do all the nonsense twice in 1 round?
There are a few rules you have to ignore to make this work:
Being able to hit four times a turn with a Booming Shadow Blade is quite a lot of nonsense. :-)
Thanks for the info.
Gruntler is right but there are also a few other things you are ignoring if you allow this:
Reference in the first paragraph of the Sneak Attack feature (emphasis mine):
Beginning at 1st level, you know how to strike subtly and exploit a foe’s distraction. Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.
Notably, it says "per turn" and not only during your turn so you can still use it if you get an Attack of Opportunity as a Reaction on somebody else's turn but that's the only way to apply it more then once.
https://www.sageadvice.eu/with-twinned-spells-do-you-need-to-roll-1-attack-roll-or-two/
For the record, the only metamagic options you can use with another are the Empowered Spell and the new Seeking Spell one which both specify that they can be used with other Metamagic options but those are the only ones.