The act of killing a Yellowfang Worg Rider shows the animation of the worg dying AND it's goblin rider.
I suggest that when the combination of Worg+Rider are killed, the goblin-rider should stand up and attack with full health... implying that you've only killed the worg, not the goblin.
Perhaps make it the opposite for ranged weapons.. that if you shoot a worg-rider.. then they both fall over.. the goblin stays dead, but the Worg gets up and attacks.
(this could also be an used with polearms as well for extra flavah!)