Can 2010 MacBook Pro power 2 daisy chained FW800 external HDs?
Yes - I have routinely used up to 4 self powered devices one one FireWire port.

Tom's hardware report that at idle most laptop sized spinning hard drives use less than 1.5W idle and less than 4W at peak - so firewire should start to run out of power between 8 to 10 drives chained together assuming your mac is putting out at least 80% of the design power and they don't all spin up at exactly the same time.