Just a guess here but if IRS L fed both pitch and roll channels of FCC A and the equivalent with IRS R / FCC B, then a single IRS failure would mean all your FCC eggs are in one basket as such, whereas under the setup you described above means that whilst a single IRS failure does affect both AFDS, a subsequent failure of an FCC still leaves either a pitch or roll channel operative. Working backwards, if the IRS was paired with a FCC in both pitch and roll, then a FCC failure would leave it's respective IRS with nothing to talk to despite being fully operative, and so once again under this "crossed" configuration it affects both IRSs but there's still redundancy built in. That's my 2p anyway, perhaps someone more knowledgeable than I can straighten it out? I think I've confused myself now!