2 Pre-Outs?

I have a high end preamplifier (which has 2 balanced pre outputs) connected to two stereo amplifiers, both J. Rowlands 112's. Each amp is connected two a pair of 8 Ohms loudspeakers with fairly equal efficiency ratings.
When I play a source (digital or analog) both loudspeakers lose their inherent sonic virtues, particularly gain. When I play each one individually(by turning off one or the other amp.), each pair performs flawlessly.
Should I increase the gain on the preamp?
Without actually seeing how you've got things hooked up, my first thought is that somehow or other, one amp-and-pair-of-speakers combo got connected out of phase to the other group.
Are you stacking?
Most speakers will not sound good played simultaneously with annother pair.
Most speakers introduce their own phase shifts throughout the spectrum. It would be rare that two models, from two different manufacturers, with two different efficencies would have the same phase shifting characteristics. Regardless of phase going into the speakers you are hearing serious cancelation.

Good luck!
I have done experiments with 2 pairs of speakers playing together before. It can work but you have to really REALLY work at getting them in the right spots so as to not get wave interference - which sounds to me is the problem you are having. In my experience, just half an inch of toe-in can make or break it. If you have them close to each other, it will be much easier. I don't think your preamp is the issue - just speaker placement and/or compatibility.

It is doable of course, otherwise home theater would be a crock of s, but stereo imaging demands much more accuracy in your speaker placement.
There is a large difference between home theater/surround sound and two pairs of speakers simply playing the same information. HT involves intentional differences in phase and timing for matrixed surround sound and discrete channels of information for DVD's.

Otherwise we could simply use the B-speaker outputs of a stereo receiver to surround ourselves with sound.