A few hints are in order, I believe:
1. I was referring to hosts appearing on each other's shows as players, not just cameos, which rules out the "Stumpers"/"50 Grand Slam" scenario (though that was a clever guess).
2. One instance of hosts appearing on each other's shows in back-to-back weeks happened in 1973 (actually, it happened twice that year, with the same hosts), and the other happened in 1980. The instance of the two hosts appearing on each other's shows in the same week happened in 1975.
3. None of the three shows mentioned above are "Pyramid."
4. There are six different hosts involved in the scenarios mentioned in item 2 -- no overlap.
Brendan