One creature is affected by Thunderous Smite, both affected by Booming Blade
The wording on Thunderous Smite is pretty unambiguous:
The first time you hit with a melee weapon attack during this spell's duration, your weapon rings with thunder...
Regardless of the circumstances of Booming Blade, and it being twinned, only one of those melee strikes will be allowed to benefit from Thunderous Smite.
There's no restriction on Booming Blade being used with Thunderous Smite, though, and Booming Blade is perfectly viable to use with the Twinned Spell metamagic, because it satisfies the requirements of only affecting a single target. So this is still a very powerful combo, even if you can't apply the smite damage to both targets.