Gain matching in a biamped system.

The two amplifiers, I suppose they are different, should not differ too much in their sonic characteristics.
To match the gain, use a sound level meter and fix an attenuator to correct level from the more efficient amp.

Depends on your tolerance for the difference in respective sound levels. Electronically it makes no difference. Personally I wouldn't settle for less than a perfect match, but some folks might like a bump up in the lower end of 1 or 2 db's.