This is how mediaportal handles channels:
A channel has one or more tuningdetails which store how and at which card to tune the channel. So if you have a channel at for instance DVB-C and DVB-S, you can have 1 channel with a dvb-c tuniningdetail and a dvb-s tuningdetail attached.
Or you can have the same channel on different frequencies in dvb-s, so you have 1 channel with 2 dvb-s tuning details attached.

Now if you are going to edit or add channel you can only edit one tuning detail of each card type, and if your are editing you can not even add a tuningdetail for a new card type. It is also not possible to delete one tuningdetail.

I would expect a screen with a list of tuningdetails per channel which you can choose to edit / add / remove. Perhaps this needs to be in the add / edit channel screen.

See: http://forum.team-mediaportal.com/mediaportal-releases-315/0002853-adding-editing-channels-does-not-match-mp-channel-handling-81812/

Patch to finish the partial (too early) commit here: http://forum.team-mediaportal.com/mediaportal-releases-315/finish-0002853-adding-editing-channels-does-not-match-mediaportal-channel-handling-84570/