Koon,
I currently have 3 SINGLE TUNER dvbs cards and one PVR150 cable card...
Haven't seen a DUAL TUNER DVBS, and I've been in it for a while...
Main problem is with dual tuner, you'd have to run 2 coax lines, to a switch, either tone, voltage, or Diseqc.. Seems a bit complicated for the average newbie trying to set up dual sat...
if they attempt to simply 'jump' the tuners, it'll fry the card if 1 tuner is 18V, and the other is 13V...
Best way I can suggest, is to get another PC, configure mythtv on it, and slave it(or run is seperately) off the linucmce myth.. That's what I've done. (basically)
I've actually got 6 receivers.. 3 dvb cards, 3 stb's on a emp centauri 9x8 multiswitch in Diseqc mode going to dual sat dishes with dual LNB's on each... (!)
HTH,
Dan