Combining a UHF and VHF antenna is easy given the low cost and availability of UHF/VHF combiners (UVSJ). Reliably combining two broadband antennas (UHF+VHF) or two same band antennas is very difficult to do successfully with consumer grade parts. One of the only remaining sources of tuned combiners that I'm aware of is tinlee.com.
I agree with teleview, try pointing the existing antenna at the weaker group of signals - you may find there's enough signal power from the stronger group to be received well by the rear of the antenna.
If that's not successful, there are other options, with no monthly fees, to replace the TiVo. A PC running Windows 7 Home Premium is preloaded with Media Center, a TV view/record application that simply needs one or more tuners to function.
http://forum.tvfool.com/showthread.php?t=820 This and other Home-Theater-PC options can be configured to use several antennas.