I asked the designer about this and here is his reply:
The DVA Digital Preamplifier's XLR output level is effectively twice the RCA output's level. Or, to look at it another way, if you look just at the "hot" side of the XLR output, you'll see that it has the same level as the RCA output. In fact, these two outputs are the same signal. So, if you use the RCA output to drive one amplifier and the hot side of the XLR output to drive another, both amplifier inputs will be connected in parallel. This shouldn't present an issue for the DVA Digital Preamplifer as long as neither amplifier has an unusually low input impedance. (The DVA Digital Preamplifer should be OK driving loads of 2kΩ or higher.)
If you use the cold side of the XLR output to drive the other amp, the two outputs will be electronically isolated. However, the cold XLR signal will be out of phase in relation to the RCA signal.
Mary