Nothing in the rules says you can't
Casting Time: 1 reaction, which you take when you see
a creature within 60 feet of you casting a spell
Range: 60 feet
You attempt to interrupt a creature in the process of
casting a spell. If the creature is casting a spell of 3rd
level or lower, its spell fails and has no effect.
The description doesn't imply you must interrupt the same spell you are reacting to. The only restriction is that you must see the spellcasting.
Now, the Twinned spell:
When you cast a spell that targets only one creature and
doesn't have a range of self, you can spend a number of
sorcery points equal to the spell's level to target a second
creature in range with the same spell (1 sorcery point if
the spell is a cantrip).
The Counterspell has the range of 60 feet and targets one creature, so it can target a second creature via the Twinned spell metamagic. It should interrupt spells which are being cast in that very moment.