The only thing you'd probably need to worry about is temperature and acclimation. With only 2 gallons, it would be very easy for the MH to heat up your water to inhabitable temperatures. In addition, any new coral additions may not be used to being placed so close to the lighting, so proper lighting acclimation may be needed.
As long as both are covered, there would probably be no reason why you can't keep 70watts over 2 gallons, although keeping temperature down would probably be the most difficult part. Raising the lighting or using fans should do the trick, but you may inevitably have to use a chiller to keep the temps down.