UrhoKarila

Alternative scenario: Caster without the War Caster feat, wielding a shield is using one hand to cast
A shield is subject to don/doff rules, so can't be removed as a free action
Assuming a somatic-active hand can't be used for the S elements of another spell, counterspell can't be cast (without losing the original spell)

Oof.
I feel like 5e was not intended to stand up to this level of scrutiny

It's the "You attempt to interrupt a creature in the process of casting a spell" that's getting me hung up

Again, it's not the most important of mechanics to hammer out the real-timeline of how this would play out

Slipping an additional counterspell would be

- A begins to cast fireball
....- B begins to cast Counterspell
........- A begins to cast Counterspell
........- A resolves Counterspell
....- B resolves Counterspell
- A resolves Counterspell

So the CS is obviously cast before the FB resolves. So we're looking at an order of actions that's
- A begins to cast fireball
....- B begins to cast Counterspell
....- B resolves Counterspell
- A resolves Counterspell

And yeah, the turn's 6 seconds long

But Counterspell's specific about when it occurs

@DavidCoffron: I took a dive into the PF system of spellcasting in that linked question. It was a bit of confusion initally about version differences, and then bemusement that the mechanics of the game allowed for this sort of thing

"It's terribly complex, prone to failure, and can fail if you take damage but nah it's fine you can cast two at the same time they're pretty easy" is just an interesting concept