If you want to watch one channel while viewing another channel via OTA (ATSC), then you need either multiple tuners, or a dual tuner.
I'm only aware of three dual tuners:
A188 Duet (PCIe)
HDHomerun Dual (LAN)
The first two won't work for you, and the third does not come with software.
So... you will need two separate PCI single tuner cards. The best PCI tuner that can handle ATSC streams is the HVR-1600. I have one and it works great BTW. Again, you would need two of these and you will have to use the bundled WinTV software that Hauppauge makes.