Well, this isn't the way you are supposed to do it, but I can tell you what MythTV requires:
First you must set up both the coax, and the composite or svideo inputs in mythtv-setup
These can be accessed in the Mythtv-Setup > Input Connections menu item
Set the both up with a provider, and a channel change command, you can use the same one for both that you already have set for the first one
Next you either need a keyboard, or an xbindkeys method of telling the system you are pressing the "Y" key.
Understand this may cause interference, as this card only has 1 tuner/mpeg capture function. For example the coax feed is always going to be providing a signal, and may bleed over into the other inputs.
I would recommend getting a second card or a pvr500, that has 2 tuners/mpeg inputs per card, .vs the 150, 250, or 350.
Hope that helps, not sure how the system is going to take it, but that is how you would do it in MythTV.