Not sure how it works with new aircraft, but it is possible to couple at least one INS to one GPS to make sure the GPS "tells" the INS if it (when) it is starting to drift off. -As in... lets say the P-3
Our INS actually drifts back and forth, correcting towards the correct position/course all the time. What timeframe and the amount of drift before it reverts the drift; I do not know. And we do use the INS's as the primary source of attitude, heading (syntetic) and position; the P using INS1 and CP INS2.