Try putting a powered USB hub between the tuners and the PC. It sounds like you may have a power problem. Once the 2nd starts tuning, it draws too much power and neither tuner has enough power to pull in the stations.
If the TV cards are USB2 (likely), make sure that you use a USB2 powered hub.
1: Dimension 9200 - Vista Ult, 2.4GHz Q6600 Core2Quad, 4GB RAM, 160GB, 750GB, 500GB USB, Nvidia 8300GS, PVR-150, HDTV Wonder, Vbox 3560, X360, DMA2100
2: HP m376n - MCE 2005, 2.6 P4 w/HT, 1GB RAM, 250GB, 400GB USB HD, Nvidia 6200, Emuzed MAUI, X360