I remember the FGTH one very clearly. When Two Tribes went straight in at number one in June 1984, Relax, which had been steadily making its way down the charts after being number one, suddenly did a U-turn and started making its way back up again, eventually getting as high as number two while Two Tribes was still at the top.
Also, I stand to be corrected, but I think John Lennon achieved this shortly after his death. I seem to remember Imagine being number one and Happy Xmas (War Is Over) being number two at the same time.