you can "getaway" w/about 3/4 Mc but there will be degradation on both sides as you go out from center. theoretically w/a good duplexer that is properly tuned to 462/467.6375 you "could" span all the channel from 550 to 725. I have seen this done once in the past and I serviced it for a few yrs but it was 2 transmitters and 2 receivers set up as a quasi combiner w/duplexer to operate off one antenna. now to the other option 2 antennas can be done "if" you have enough isolation. horizontally at 10W you would need about 30' or so w/unity antennas. vertically you could do about 10' w/lower ant inverted.
JohnE's post in Duplexer Tuning / Distance Without Duplexer was marked as the answer