I think you'll find that the arrangement lasted for quite a few years from the late 70s to early 80s, although I can't seem to find exact dates.
I think it lasted two years - the first Radio Times I've got with that format is from May 1977 and the last is June 1978, but I think it started in late 1976. This was also the period when Today was only broadcast in two half-hour chunks, from 7 to 7.30 and 8 to 8.30, the reason being that apparently the controller of Radio Four was obsessed with the idea that current affairs programmes were too long and if they were shorter they would be more concise and hard-hitting, so as well as that he cut PM from an hour to forty minutes (with a filler show called Serendipity at 5.40) and The World Tonight from 45 minutes to half an hour.
From 6.30 to 7, and again from 7.30 to 8, there was a "programme" called Up To The Hour which featured all the non-news stuff from Today like the sport and Thought For The Day, and even some records, presented by the day's announcer. Everyone hated it because it was such a pointless show, there's a famous story of Peter Donaldson introducing it by running through what was on all the other stations and then saying "But sadly, here on Radio Four it's Up To The Hour". And the Today team were of course devastated because it had been cut to nothing.
Anyway, late 1978 it was back to one long show again, and all done from London. I suppose the idea was to make it less metropolitan but it just sounds awkward.