Actually I was going to say why I wouldn't allow it but, I guess I would allow the secondary effect on a second target only when a sorcerer twins the soell that's fair. And as a DM, I would allow Green Flame blade twinned the way I said earlier.You have this exactly backward. GFB cannot be twinned because it has more than one target. Booming Blade has one target and doesn't have a range of self, so it can be twinned.
http://www.sageadvice.eu/2017/04/28/is-is-possible-to-twin-spell-booming-blade/
http://www.sageadvice.eu/2015/11/10/can-you-us-twinned-spell-with-green-flame-blade/
Sent from my VS995 using Tapatalk