When a barbarian uses knock back, can he move two opponents as normal with a second maneuver roll? If so, does the second target take damage equal to the barbs strength mod?
Yes, on the first one, this IS a Bullrush.
No on the second one, the damage isn't part of the Bullrush per se, they're distinct effects of the same special attack. Of course, if they fall off a cliff from being Bullrushes, there's usually damage from that :-)
There is the Raging Throw power that does extra damage to the first Bullrush target AND the people they knock into, and that should be able to apply to Knockback Bullrushes. I'm not sure why they call it Raging Throw though... Kind of confusing name IMHO.