Get one of these http://www.smarthome.com/7717.html - works wonderfully. I use one to pass OTA and both TV tuners over, and I used this when I had 2 single tuner boxes. The only real downside is the UHF modulators are mono sound only.
Your other option could be to just get individual UHF modulators (also available on that website)
Your other option could be to just get individual UHF modulators (also available on that website)