That is the thing. To me, ray of frost
is as ambiguously written as booming blade.
It has all of the same components and the same logic works to say that you don't get a 2nd attack.
We can fall back to natural language. In which case, both spells clearly have 1 target. And the minimal changes you'd do to each spell to give them a 2nd target and have them work are clear
in both cases.
If we use restrictive reading and "only add an additional target, no other changes", then the extra target is useless in both spells. You don't get another beam, you don't get another spell attack.
A frigid beam of blue-white light streaks toward a creature within range. Make a ranged spell
Attack against the target. On a hit, it takes 1d8 cold damage, and its speed is reduced by 10 feet until the start of your next turn.
to turn this into a two target spell, you do this:
Two frigid beams of blue-white light streaks toward
two creatures within range. Make a ranged spell
Attack against
each target. On a hit,
that target takes 1d8 cold damage, and its speed is reduced by 10 feet until the start of your next turn.
Booming blade reads as follows:
As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails.
On a hit, the target suffers the attack's normal effects, and it becomes sheathed in booming energy until the start of your next turn. If the target willingly moves before then, it immediately takes 1d8 thunder damage, and the spell ends.
To turn this into a two target spell, you do this:
As part of the action used to cast this spell, you must make a melee attack with a weapon against two creatures within the spell's range, otherwise the spell fails.
On a hit, each target suffers the attack's normal effects, and it becomes sheathed in booming energy until the start of your next turn. If that target willingly moves before then, it immediately takes 1d8 thunder damage, and the spell ends on that target.
Both require rewording to handle a 2nd target. The rewording in each case is no larger than the other. I guess you could drop "
on that target" and have only the first moving creature take the damage.
In 5e, the spell picking a creature causes it to be a target of the spell. Not "I cast a spell and pick targets then read the text". If the text picks creatures? Those are the targets.
If you don't do that rewording, both spells don't make sense with 2 targets. If you do do the rewording, both spells make sense with two targets.
I understand your position -- that somehow, the melee weapon attack makes these fundamentally different -- and it seems to be grounded in the idea that sorcerers cannot make two weapon attacks as part of the attack action without using a spell, so they shouldn't be allowed to do it as part of a spell.
And if there was a balance implication of letting a sorcerer who spent resources on learning booming blade, built up their attack stat, and burned sorcery points to getting a 2nd attack booming blade attack, I might houserule that booming blade doesn't work with twinspell.
But I'm not seeing the problem with treating it like any other "you attack the target" spell being twinned and making the same minimal modifications to make it make sense. Because that seems (to me) to be the intention of twin spell, if not how it was worded when read strictly.