It makes sense to try to put the LO in the center of the 2 frequencies, but indeed if you want to avoid having some DC offset, and you can sample fast enough, parking the LO outside your band of interest is a good idea.
When using NCO for faster frequency switching, you may want to check Frequency hopping transmit [FIXED] - #13 by KarlL as well as Max NCO, relation to sampling rate, and NCO effect for more general info on what can be done with NCO.