The best thing is to use a rotor. Another option is to use a product like Jointenna from Channelmaster so you can combine the antennas without producing terrible multipath problems. You have to order it for the specific channel desired i.e. in your case the NBC channel. Basically it filters out all channels but one for one antenna and filters just the one channel on the other antenna before you join the signal.
There is info on these products at channelmaster.com and I think they're available online at Stark Electronics (probably other online sources, too).
Be sure the antennas are spaced adequately (50 inches should be o.k.) to prevent unwanted interaction.
P.S. You could also run two seperate downleads into an A-B switch and simply switch between them, but if you're using a preamp it may be impractical to have to duplicate the whole setup.