Calculating the damage of a hit inherently also calculates the damage of the ailments it will inflict. This combines damage from multiple calculated hits in the exact same way as Dual Strike, Cleave, and (most similarly) Explosive Arrow.
You hit each enemy once each time you wound them, calculating the damage of the hit (including damaging ailments) but not applying that damage. You are still hitting, so do get all on-hit effects that don't require damage.
At the end, for each enemy, you hit them an additional time, causing on-hit effects again, and dealing the combined damage of all the wounds applied to that enemy as a single hit. If those hits calculated damaging ailments, the damage of those ailments is merged the same way. If some of those hits were crits, then the critical multiplier will have applied to those specific hits when they calcualted, and will not apply to others, but if any of the combined hits was a crit, the combined hit counts as dealing a criti...