"Under Pressure" is a pretty strange song, as far as duets go. Freddie Mercury and David Bowie aren't singing to each other or even together most of the time, almost as if they're not even in the same room.
There's a reason for that, and it's because they weren't.
The song came out of a jam session in Switzerland, where Queen had settled down to record Hot Space. Bowie happened to be in town, so he stopped by the Queen Compound to say, "Hi," because you could just do that when you're David Bowie. They started jamming, recording whatever songs they could think of, and also doing just so many drugs. In other words, it was a perfectly average Queen and/or Bowie party.
Then Queen's bassist, John Deacon, started playing the bassline that would catapult Vanilla Ice to thoroughly unnecessary stardom ...
... and everyone lost their goddamn minds. They decided they needed to write a song right then and there, and ... went out for a pizza instead, during which Deacon forgot the bassline. Luckily, it came back to him, and once they'd laid down an instrumental track, Bowie had a stroke of cocaine-fueled genius. "Okay, hear me out," we like to think he told Mercury. "You go in that booth, I'll go in this one, and we'll just sing whatever comes to mind. It's gonna rule."
He insisted that they record in total isolation, without hearing what the other person was singing. That's why it sounds like two different songs: It actually was. That's also why Mercury's part is mainly gibberish. He was coked out of his mind and put on the spot by David freaking Bowie. None of us could bring ourselves to make sense under such conditions, but the difference between Mercury and us is that even his nonsense is genius.
After nearly 24 hours investigating the line between brilliance and madness, everyone crashed, and when they came to, they set about the daunting task of trying to string all these random tracks together into a song. It worked, but the real takeaway is that somewhere out there, someone is just sitting on a recording of Freddie Mercury singing "All the Young Dudes" and all the other songs Queen and Bowie jammed on before they decided to get down to manic business. Whatever they want for them must be terrible.