You can unlink two channels by Pressing & Holding the SELECT button on one channel and Pressing & Releasing the SELECT button on the other linked channel. (after unlinking, hold down the INPUT button you're working with, the channels routed to that input will flash)
Remember, when two channels are linked, the fader on the ODD channel controls volume on both channels, the EVEN channel fader does nothing.
And, Yes, there was talk of INPUT A&C being best suited for odd channels and INPUT B&D Best for even channels. (I think that's mentioned in the manual)
BUT, with single channel recording, I don't think it makes any difference which input is routed to which channels. I'll do a test at home tonight, just to be sure.
Hope this helps
P.S. - I think when you start a new song, channels 5&6 are linked (by default) and panned hard left [Channel 5] and hard right [ch 6]. They can be unlinked, using the method above and the pans certainly can be centered.