Reactions can be performed when the "Trigger" Is met. If a player holds action to attack a goblin that gets within striking distance, that is the trigger and it is the player's choice to act on that trigger.
A trigger happens only once and any applicable reactions must be decalred, else that creature misses the applicable trigger.
If a creature casts a spell, a trigger for a counterspell occurs. If two creatures choose to counterspell, they both must declare they cast the spell as a reaction to the trigger. Regardless of the first counterspell attempt, both creatures cast the spell to disrupt the enemy.