It's not advisable to stack a preamp and power amp together. The transformer in the power amp has a rather large, strong magnetic field that can cause electro-magnetic interference (EMI) with the circuits of the preamp. Preamps, which handle relatively weak signals (compared to the power amp), should be placed some distance from the power amp -- it does not need to be separated by a wide distance, since the electro-magnetic field weakens rapidly. I haven't seen any specific guidelines in audio mags or books, but I generally try to keep the preamp at least 6" away from the power amp, and preferably a foot away (if the equipment rack allows for this). Even placing the power amp and preamp on separate shelves with a shelf in between the two units can make a significant difference.
As an aside, phono stages should also be kept away from power amps -- the phono stage carries an even weaker signal than the preamp, so the phono signal is particularly susceptible to EMI.
If you connect two pairs of speakers to the single binding post on your amp (wiring them in parallel), you will effectively be reducing the speakers' collective impedance to half the normal amount (i.e., two 8-ohm speakers connected to the same binding post will yield a 4-ohm resistance). Since virtually all speakers have an impedance curve that dips below the nominal level (many nominal 8-ohm speakers dip as low as 4 ohms of impedance), the further reduced impedance could be a problem for your amp.
