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.
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.