On the same transponder it should be possible. But I'd try leaving the B unconfigured. After all, even having only one tuner it is possible to record from the same transponder. Obviously, only FTA channels, anyway. Crypted one can't be used at the same time neither by many tuners nor if in the same transponder because you can't simply decrypt two channels at the time by your smartcard. It's a limitaton of the smartcards.