Because you already own a Tivo you may want to stay with that product...
If so, yes, to take advantage of two antennas, a second Tivo would be needed if you want to record from both sources.
However, there are other competing products,
http://www.epvision.com
http://www.channelmasterstore.com
http://westerndigital.com
http://www.silicondust.com
http://forum.tvfool.com/showthread.php?t=820
http://forum.tvfool.com/showthread.php?t=2882
I have multiple antennas. One is primary and usually serves everyone's interests, therefor it's cabled to all the TV's in the house. I don't use an A/B switch or rotator. The remaining antennas terminate in at least one network attached tuner, making it possible to record from any antenna without conflict. The best PVR/DVR for me has been a PC running XP. The recording files are stored on a networked drive, available to any PC in the house. If we want to watch a recording on the big screen, the laptop has an HDMI port that feeds the big TV screen.