Within MP you can disable one of the tuners, try disabling the good tuner and see if the other tuner is working. If the other tuner works with the other one disabled presumably this eliminates a signal problem. If you have channels mapped to both tuners then sounds more like a faulty card, but unfortunately with htpc there are so many factors going on that nothing is certain.
to disable one tuner goto TV-Server/Configuration/TVServers - MediaPortal Manual Documentation
look at the 2nd pic, and toggle the tick boxes to enable/disable.
to disable one tuner goto TV-Server/Configuration/TVServers - MediaPortal Manual Documentation
look at the 2nd pic, and toggle the tick boxes to enable/disable.