I'll answer your question in the next paragraph. But first let me be sure that you realize that if the amp isn't designed for bridging, putting the same information into both channels of one amp, and then connecting a speaker between its two "hot" outputs, will result in essentially zero sound from the speaker (since there will be essentially no voltage difference between its terminals). A properly bridged amp will invert the phase of one channel, and will do that without introducing significant time delay differences between the channels other than the phase inversion.
To answer your question, though, feeding one output channel of the preamp into two input channels of a power amp should not be a problem, provided that the rated input impedance of the power amp is considerably higher than the output impedance of the preamp. With two channels fed, the effective input impedance of the power amp will be half what it is normally.
Also, it would probably be best to do the split at the preamp output, so that you are in effect bi-wiring to the power amp. If you just run one cable from preamp to power amp, and then a short jumper to the other power amp input, the lower combined input impedances of the two amplifier channels might magnify any cable-related degradations of the signal, particularly at high frequencies.
Regards,
-- Al
