Hmm... my initial response would have been that you couldn't convert the double, but after looking at pictures on dw's website, I'm not so sure.  It looks like there's a hole for a beater as well as a mount for a spring on the slave, but I don't think I would buy a double pedal with the intention of splitting it into two singles, but you might try it if you already have the double.

As for heads, you probably want double-ply heads for durability, but to keep a more open sound, don't get ones with a muffling ply, you'll want something like Remo Emperor, Evans G2, or Aquarian Response 2.  The guy from Trivium uses Emperors over Ambassadors.

I do not think it can.

Nope you can't do that as far as I know. However you can take the beater and send it over to your second pedal. How ever it still wouldnt be as smooth as having two single 9000's. For heads, I play Aquarian Performance II heads. They sound fantastic and are great for metal!

I've owned 2 double pedals in my life, a Pulse and a Yamaha. Both of those were able to be converted into 2 single pedals so I don't know why this one wouldn't be. As for the heads I like my Evans G2 coated heads. They're nice and warm. Perfect for the sound I want.

If you're planning on buying the 9000 to split, i might suggest looking at the axis pedals which are absolutly easy to split.

Heads: I also play tech metal of a fast sort, I am currently playing an old pearl export set (yes, by choice).  It sounds really darn good if you know how to tune it up. I'm playing evans G2 coated on the top of the toms and evans black reso on the bottom.  This gives me a nice full sound with some sustain, but not too much to become muddy at speed.  On the bass I played the EMAD2 forever, and I loved the sound (a healthy thump with some body behind it) but the plastic ring kept breaking off.  I have recently switched to the SK1 and I'm quite happy. Remo makes a  new head with external damping that looks pretty interesting.

Let us know what you end up with!