Communicate, Plan, and be Consistent. Those are the three things I always do whenever I am in the woods with anyone. We tell each other how we're feeling (physical fitness wise) repeatedly throughout the day, we have several times/places to stop at or check in that we know of before the day starts. And finally, we stick to the same M.O. for hiking.

I always have a whistle, but often hike with people who do not carry one - just like I always have a compass, and often hike with people who do not bring one. If you know who you're hiking with, plan ahead, and have the same practices from hike to hike, I see no reason a married couple (or some buddies who know each other well) need to stay within earshot on a trail.

Off trail is another matter entirely.