stacking components

My Rotel RC-1070 preamp manual states not to stack on top of amplifier due to possible noise interference. Can this be done or should I stack the amp on top of the preamp. Also, mr amp (Rotel RB-1070) has only one pair of speaker terminals. Is it still possible to hook up 2 pair as long as they are both 8 ohm? What type of connector do Iuse for the 12V trigger connection? Thanks for your patience, I am a noob when it comes to mid-fi components.
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.
Do most lower end mid-fi amps have only one set of speaker binding posts? I was a bit surprised to see this when I was reading my manual. A way around this would be to use a speaker controller box?
Hooking up additional speakers in parallel means the second speaker will connect to the terminals on the first speaker. For selective switching between pairs, yes you'll need a selector box. Both pair will connect directly to it.