That's how I see it, too.Rich H wrote: "... during the following round, should you or your companions hit your foe with a Piercing Blow..."
So, in the next round of combat you and your companions all get a chance to take advantage of the weak spot in the worm's armour that has been uncovered.
Consider that, during the round that the Gandalf was rolled, in fact all attacks are not a single blow.
That is, although Burin and Beor might be in different Stances and roll their attacks in a certain order, that doesn't exactly mean that Beor swings one blow after Burin does. Both are swinging blows and dodging various times during their round, and the roll only represents the added outcome of all their actions.
Therefore, it's not until next round that the Troll has his soft point uncovered and Burin's companions are able to notice and prepare their attacks for it.