If you have 2 equivalent tuners, then MC is supposed to use the other tuner. If you are recording back to back shows on different channels, sometimes the network defines the timing of 1 of the shos such that there is an overlap between the 2 shows and thus MC needs the 2nd tuner to make it work.
For example:
Show A on channel A scheduled for 8:00-9:01
Show B scheduled for 9:00-10:00
In the above example, you will need 2 tuners during the 1 minute overlap. If you only had 1 tuner, then MC would alert you of a conflict and not record 1 of the shows.
If your tuner is not behaving properly in MC, then I would see if there are newer drivers for your tuner card. It is possible there is a bug in the ones you are using.
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