I do not think it matters if it is your spouse or a group- you need to stay reasonably together. At least have several designated "meet up" points during the day - the logical place is at every trail junction. Here is my concern. Everything seems fine until it goes wrong. You get to the end of the day and the other person is missing. What do you do? Hunting for someone when you are already done for the day and tired is not very appealing. Sitting there and worrying is not appealing. And what if the missing person has the tent or stove? Yes, staying together is trying at times, but for me the alternative if things go wrong is much worse. You can always slow down the fast person by giving them more gear to carry. If you simply want to hike your own pace, then find someone with a matched pace or go solo.

If you choose this method of hiking separately, then each person should be self sufficient with food and shelter and you should have a solid plan on what to do if you get separated.

I think that assuming that simply by carrying a whistle you avoid the chance of getting seriously separated is a false sense of security. PS- I have seen some groups who carry hand-held radio communication devises- often families with kids. this is better than a whistle.