I'm trying to understand the effect of this situation. It seems that the preamp will be driving two loads and therefore the input to any one amp will be less than it would if only one amp were driven.
Either that or the output of the preamp will be more than if only one amp were driven

More in terms of output current, at any rate.
It's a simple division between the output impedance of the preamp (a few hundred Ω) and the input impedance of the amps (many kΩ). More amps in parallel - less effective impedance in the lower branch of the divider. The division ratio is still close to 100%, so the difference isn't big, but current draw is doubled. Moreover, output impedance of the preamp is likely in >99% dominated by protection resistors and if there are two sets of outputs built-in, chances are that each has separate protection resistors and there are effectively two dividers in parallel, completely independent of each other.
According to the manufacturer of the preamp this is ok.