I think it has something to do with the fact that each Autopilot gets Pitch and Roll data from Both IRS's. If 1 IRS is U/S with the Attitute transfer switch in Normal then 1 of the signals will be errornous. If you Select the Attitude Transfer switch in the position "Both on L" or "both on R" (depending on which IRS is at fault) then The autopilot will function however it will only get Attitude data from 1 IRS ie there is no redunduncy in case of a problem with the other IRS.