Of course, these days it may be possible to do it over a high-speed connection...
Still it would be impossible to get the timing right simultaneausly on both sides of the Atlantic. The audio/video delay from one side of the Atlantic to the other is still some 2-3 seconds.
What you could do is something like this: Make Jagger sing live in the US and transmit the signal to the UK where Bowie can join in fairly simultaneously. Like in Karaoke. This would give the European stadium audience a Jagger/Bowie live duet. Of course you cannot send back Bowie's signal into Jagger's ears because he would have to wait for what he gets from Bowie while Bowie is singing to what he hears from Jagger first. The Death Of A Circle. So the Jagger audience can only get Jagger live, without Bowie.
But here is plan B: Take the mix signal from Bowie in London (him singing to Jagger's signal from US) and send this to TV sets all across the world. So you would have the Jagger/Bowie live duet everywhere on this planet apart from the stadium where Jagger is singing himself. They could have done that in 1985.
And I want to believe In the madness that calls 'now'