Pre-Amp with A/B Switch?

Instead of pre-amp with dual outputs, I use a Bryston 2-way speaker switch box. You can select both, A or B, or none. It also includes a low impedance headphone jack. This is a nicely made product with gold plated switch contacts. I drive some serious current through it with a Bryston 14B SST amplifier into Snell Type A/II and Cambridge Soundworks Tower loudspeakers and have not heard any audible effects.

http://www.bryston.ca/2switch.html

If you'll only have a pre-amp with A/B switch, you will still need a second amp to drive a second set of speakers.
Some McIntosh pre-amps have the A/B switches. McIntosh integrated MA6400 and MA6500 can control 2 sets of speakers.