Guess it varies by type. On the 744 normally the three IRUs send their position to the FMC and it "triple mixes" the aircraft position. And the normally the left FMC is the master, feeding the right info. Above 84 degrees each FMC gets its it's own IRS and you lose the triple mix. So you could have a difference in the nav displays left and right. Above 88.5 degrees GPS is inhibited and you are unable RNP. The GPS will start updating again passing 88 degrees and below 83.5 degrees the IRUs triple mix again.