That will most likely work fine, and should give superior results to horizontal biamping, for the reason Jjrenman cited and perhaps also because it would make it possible to use shorter speaker cables.
The reasons I say "most likely," though, (and these reasons are equally applicable to the horizontal and vertical biamp configurations) are as follows:
1)Chances are that the two sets of output terminals on the preamp are simply wired together inside the rear panel of the preamp, rather than being driven by individual buffer stages. If so, the output stage of the preamp will see a load impedance equal to 1/2 of the input impedance of the power amp. I found a spec indicating that the output impedance of the preamp is "less than 600 ohms," but that is probably at mid-range frequencies and could be much higher at deep bass frequencies. And I couldn't find a spec on the input impedance of the power amp. Given that the power amp is tube based, I'd feel fairly confident that 1/2 of its input impedance will be sufficiently high in relation to the output impedance of the preamp to not result in sonic issues, but that can't be said for sure without having more complete specs.
2)Assuming that the two sets of output terminals on the preamp are not individually buffered, the output stage will be loaded by the sum of the capacitances of two sets of interconnects. That most likely won't cause any problems, but if the interconnect lengths are particularly long (e.g., in the vicinity of say 12 feet or more, each), choose cables having low capacitance per unit length (e.g., 25 pf/foot or less). A slight rolloff of the upper treble might result if the cables are long and have high capacitance per unit length.
Regards,
-- Al
