You can use the preamp-out connections on the back to feed a power amplifier, thereby using the 850 as a preamp only (volume control, tone control, source selection, etc).
You can then run all four speakers from the power amp, or you can choose to run one pair from the power amp section of the 850 and one pair from the second power amp. In this scenario, you would want the separate power amp to have its own gain controls, so as to be able to match the output of the speakers (the pair connected to the 850 and the pair connected to the separate amp).
Note that the the SX-850 can handle a 4 ohm load (2 pairs of 8 ohm speakers). If you choose to go with a separate power amp, make sure it can accommodate the intended speaker load (the combined impedance of multiple speakers). Also, if you go this route, I would suggest you move up significantly in power capability. A doubling of power provides only 3dB in headroom/loudness.
If you are contemplating this change only because the impedance load of your particular speakers is lower than the 850 can handle, another alternative would be a standalone speaker selector with impedance protection. These units are inexpensive, and generally use resistors or autoformers to assure that the load doesn't drop below a specified value (usually 4 ohms).
One last point...if you end up using a separate power amplifier, note that many of them (most) do not provide speaker selection capability. In fact, most only have one pair of speaker terminals. You can of course hook two speakers to each terminal (again, assuming the amp can handle the combined impedance), but both pairs will always be active.
The solution to this issue is a separate speaker selector box (same as described earlier, above). If impedance is not an issue, use a selector box that does not include impedance protection...it's always good to keep additional components out of the circuit if possible.