Let's say I am a level 5 sorcerer and I have already cast shadow blade. And I want to do the following on my next turn:
Quicken green-flame blade for 2d8 (from shadow blade) + 1d8 (GFB dmg) on the main target and 1d8 + spellcasting modifier on the creature next to it.
Then as my action I Twin the GFB again, and I make the first attack on target 1 and second attack on the target next to it, which allows me to do basically double damage from above calculation.
Is this allowed by RAW? If not why?
Best Answer
Green flame Blade can't be twinned
For a spell to be twinnable it needs to only target one creature. As clarified in the PHB errata:
Green flame blade is able to target two creatures; the one you attack and one next two it. (See also developer tweet on the matter who agrees with this.) D&D 5e does not define targeting as well as it maybe should. If you would like to read more words on this and/or argue it, I direct you to the questions What counts as a target for a spell? and ask you not to argue it in my comments.
The rest works
To go through the logic: Green flame blade requires you to attack with a weapon you are holding and shadow blade explicitly creates a weapon (as opposed to say flame blade). There is also no limitation on casting two cantrips per turn, only to casting a spell of 1st level or higher on the same turn as casting a spell of 1st level or higher as a bonus action.
This may be efficient, but it isn't majorly broken though, it's costing you a second level spell (which you have to maintain concentration on) to set up and two sorcery point every turn (to quicken the cantrip).