Wizard NPC Zed casts a Fireball at 4th level. PC Alice casts Counterspell at 3rd level to prevent this. Unfortunately she fails the ability check to counter the spell. PC Bob is also within 60 feet of Zed, and has Counterspell prepared and a reaction available.
Can Bob decide, after it's known Alice's Counterspell failed, to cast his own Counterspell?
Further, is the answer at all dependent on the casting time of Fireball (1 action)? e.g. would the answer be different for a 4th level Shield spell, due to the "shorter" casting time?
This question isn't:
"Can a casting of Fireball be targeted by multiple Counterspells?"
Clearly there is nothing to prevent that - the question is whether the multiple Counterspells have to be cast simultaneously, or can be cast one after the other.