There's a better way than "blindly" chopping your main in a two-out scenario. For one, if you are unlucky enough to have the main and reserve risers entangled, chopping it can be certain death.
I've had the luck to sit through Jim Cowan's Dual Out briefing recently, and his breakdown of possibilities and options is top-notch. It's also on youtube, so I highly recommend everyone to watch it.
As for the specific effect of a much larger reserve than main, I would guess that the reserve is much higher over your head (providing more lift) while the main flies a lot more twitchy. My gut feeling would be that you might have to rein in the main flying it in deep brakes or rears (if half-brake is still set) to keep the two cooperating. Cutting the main away IFF it's not entangled would probably be your best bet, secondary to keeping both canopies nailed together down to landing.