Searching for a book from the 70s

I do hope that my gransnet friends are able to help me. When I was in secondary school in the early 70s, our headmistress used to read a chapter of a book every day in assembly. One book stays in my mind. I cannot remember the author but, it is about a young couple, in his ‘now’ the wife is killed in a car crash, however, in her now it is the husband who is killed. They employ a solicitor to help with the estate, and one day the husband writes an entry in his diary, the following day, there is an entry below his from the wife. It goes on like this and eventually the solicitor puts his entries in the diary too. Cutting it down, one day the solicitor goes to the house, the house is closed up, but there are entries on the same line. Does this make any sense to anyone? Does anyone know what this book is and who wrote it?