Please don't make it too complicated - and not more complicated than it could be!
My proposal: Add a check-box (or else - what is most appropriate) in the same "device selection window" where you select your device. On that check-box should be written something like: "ask for updating channels?" If it is selected, you get the question whether to rescan channels (or remove unused) as provided now (I don't know whether every device is asked separately) - and if it is not selected, you don't get that question and there will be no channel update/removal - and all is kept as before.
So add just a criterion whether that question should appear or not - not more and not less - and best thing is to add that in the same window where you choose your device (of course in both views - simple and advanced). That would intercept the "problem".
BTW:
Juergen wrote:Theoretically this should be possible, if separate (dynamic) lists were used by device, showing up or getting hidden by device state.
Even then, a common favourites list might cause trouble. Stations from an inactive device would have to be hidden or deactivated there in.
Not really easy to do, I suppose ...
To be honest: that's a different problem ... (I also complained at least once before - i.I.r.c.) ...
Until now (some versions before - 6.50 or else) it was so that when you added new channels, they were put in all the "hardware lists" i.e. having two lists here with the pinnacle quattro stick (DVB-C and DVB-C), I had the DVB-T channels also in the DVB-C list and vice versa - and that's no good - specially when they are on the same channel/frequency. So treating the devices separately would be really a nice thing - but I treat that as a different task than the question whether to scan device or remove channels.