Split packs like we're talking about are OK, but modifying packs to accept two circuits is not a good idea. I don't like the fact that you could possibly have 240volts in a pack designed for 120 volt operation.
As for your question about wattage when dimmed:
I'm not sure how the wattage is exactly ( I might just put a meter on a
dimmer to figure it out), but if you have a 1200
watt load on a 1000 watt dimmer and leave the
channel at 70% it normally will stay on.
I DO NOT recommend ever overloading a dimmer past its stated capacity. If someone brings everything to full all at once you risk putting a huge shock load on the building electrical system.