Simon Price on Jay-Z and Justin Timberlake's Legends of the Summer: Is one stage big enough for two Olympic egos?

Mr Carter and JT chose the site of last summer's glories to try out their collaborative show

Superstars: The evening is a hits jukebox with a full live band, like an old-style Vegas revue (PA)

One mock-historic handshake, and the Legends of The Summer grand premiere is underway. Not on a red carpet, but on an outsized rectangle of butcher's grass in what is, thanks to the combined efforts of the Luftwaffe and Locog, the flattest brownfield site in London.

For two nights, Jay-Z and Justin Timberlake have played their individual headline shows, each making a cameo at the other's concert. But this is the big one: ahead of a full US tour, this is the first try-out of a collaborative two-man show, with a full live band on red podiums, and backing singers in matching scarlet evening wear, like an old-school Vegas revue.

It means, for example, that when Timberlake orders "drums!" after the spoken interlude in "Like I Love You", there will be two drummers and two kits to oblige. Except there won't, because the song is cut short before we even get to that, and segues instead into the concussed psychedelic future-soul of "My Love". Such is the rapid-fire nature of the set list.

After opening with the Nirvana-interpolating joint single "Holy Grail", the two superstars throw themselves into a hits jukebox in which their respective oeuvres are jammed together to accommodate both egos. Uncomfortably, at times. Timberlake's immaculate "Rock Your Body" is not necessarily improved by a Jay-Z rap. Arguably, few things are. Sadly, even fewer people have got the memo.

That song's promise to "have you naked by the end of this song" isn't allowed to linger, whipping straight into the piano glissando from the Jackson Five's "I Want You Back", which in turn gives way to Jay-Z's "Izzo (H.O.V.A.)" Upon which, the pretence of equality is momentarily dropped and Jay-Z gets to do his hits segue. Shawn Carter is a strange character – obviously not devoid of talent, and a super-fast rhyme-spitter when he wants to be, but hobbled by a messiah complex that's outshone only by Kanye's. Jay-Z is, along with P Diddy, the epitome of rap star as CEO. He's become the Coldplay of rap, a one-stop shop for people who don't really like the genre.

There's something accidentally hilarious about a bunch of nerdy white guys screaming the line "I got 99 problems but a bitch ain't one" emphasising the B-word in their strangulated John Major voices. They wouldn't dream of talking to women like that in real life, but Carter has given them permission. It's a vignette that sums up the difference between the two stars: Justin gets guys to shout "ladies!", Jay-Z gets them to shout "bitch!"

The two are on stage together for such a long time that they need to find ways of averting the danger of looking like a spare part at a gang bang. Justin plays a perspex guitar on Jay's "U Don't Know", messily inserts the "Walk This Way" riff into "99 Problems", and jokily acts out the part of a racist cop. And between them, they urge us to "bounce!" so frequently that I wonder who's giving them a backhander: the dog food, or the fabric conditioner.

"Can I get up and do my thing for a little bit please, London?" asks JT to deafening assent, which fizzles somewhat when it becomes clear that he intends to perform unheard new material from the upcoming sequel album 20/20 Experience: 2 of 2. (Jay-Z's Magna Carta tracks are, at least, from the current UK No.1.) But Timberlake is a consummate modern pop star. He can sing, dance, beatbox, play guitar and piano, and do it all with self-effacing charm. What he isn't, though, is a balladeer, and a mid-gig section of slowies brings a slump. In his head he's probably Prince doing "Adore", but the reality can drag. It does, however, bring a moment of comedy synchronicity when Justin compares his "Pusher Love Girl" to MDMA at the exact moment that a local entrepreneur, lurking underneath the Yahoo! flagpole, tries to sell me the real thing.

Rumours have been circulating all day about celebrity guests. Only one turns out to be true: Rihanna performs "Run This Town" with, as usual, all the humanity of a Blade Runner replicant. But all is not lost. Timberlake takes back the night with the electrofunk of "Sexyback" and the 21st century Philly of "Suit & Tie", reminding you that, at his best, this guy is unbeatable. Put simply, Justin IS disco. And if you don't dig disco, you're a philistine savage who cannot be reasoned with.

Critic's Choice

The Strypes: teenage sensations breathing life back into Sixties R&B, or a retrogressive fantasy made from Paul Weller and Noel Gallagher’s old hair clippings? You can be the judge at the Academy, Oxford (Thu) and Secret Garden Party, Huntingdon (Fri). Meanwhile, evergreen dance-pop heroes Basement Jaxx play Somerset House, London (tonight) and the Kendal Calling festival (Fri).