It official: U2 is the biggest music act on the planet! The band might not go multi-platinum like it used to, and there’s been no blockbuster single since George Bush the elder was in office, but Bono and the boys just bagged some brand-new bragging rights. In early April, the group’s 360° world tour surpassed the Rolling Stones‘ 2005-2007 A Bigger Bang tour to become the biggest money-making road trip of all time. By the time the Live Nation-backed trek”which U2 launched in 2009 to support the No Line on the Horizon album”concludes in July, it will have pulled in a projected $700 million in ticket sales.

Of course, U2 didn’t do it alone. If a GRAMMY were awarded for Best Supporting Act, Muse” who opened for many of U2’s 360° dates, including the ones in Brazil that broke the Stones’ record” would have an excellent shot. (Jay-Z did the honors in Australia and New Zealand, while the Black Eyed Peas chipped in on some US and Canada dates.) Here’s a platinum-level UK band whose slow and steady trajectory in the US has been thrust further upward by key slots on the soundtracks to the Twilight films and frontman Matthew Bellamy’s romance with Kate Hudson, with whom he’s expecting a baby. Muse could sell out big venues on its own (and did, even before U2, Hudson or Twilight entered the picture), but with the group playing warm-up act for U2, it seems almost inevitable that major records would be broken.

Two superstar acts for the price of one ticket: It’s a brilliant idea that’s spreading fast. With the international economy in shambles, and so much competition on the road, the biggest stars need to offer fans more than just the greatest show on earth to guarantee blockbuster box-office business. That’s where A-list opening acts come in. U2 could sell out stadiums and arenas solo, but why not hedge its bets by bringing in big-name support to pull some of the weight?

In previous decades, most big stars wouldn’t have been caught dead with an opening act that could possibly upstage them. They usually hit the road with bubbling-under, up-and-comers, safeguarding their own star billing while, by default, helping the upstarts bring their music to the masses. But with ticket sales skyrocketing closer to four-digit figures (U2’s $250 top-tier ticket price is practically a bargain), sometimes you need more than a name and a collection of hits to lure fans. Simply put, on their own, few superstars have the drawing power of Charlie Sheen anymore. It takes two (bankable draws) to make a gig go right.

These days the relationship between headliners and opening acts (or co-headliners) is far more symbiotic. Rod Stewart and Stevie Nicks co-headlined”and sold out”the thirteen-city Heart & Soul North American tour in March and April, though she who goes on first (sorry, Stevie) is technically the opening act. Eighties teen queens Debbie Gibson and Tiffany just announced their own co-headlining summer tour (at press time, there wasn’t any word on who’d be opening), and Sade will bring fellow platinum-level GRAMMY winner John Legend along for the ride when her world tour arrives in North America on June 16 in Baltimore, Maryland. Though there is some fan overlap in all three cases, Nicks and Legend will be contributing to the financial potential of their tickets in a much larger way than your traditional opening act.

Meanwhile in the world of pop, Britney Spears and Nicki Minaj just signed a pact for Minaj to open dates on Spears’s upcoming Femme Fatale North American tour, which also launches on June 16, but in Sacramento, California. The unlikely alliance between a superstar and an up-and-comer who, at the moment, is probably just as hot, will benefit the headliner as much as the opening act. It will expose Minaj to pop fans who might otherwise know her only from her cameos on other people’s records, and it will give Spears a little bit of something that has eluded her for her entire career: street cred. It’s probably an even more winning combination than Spears and Enrique Iglesias, who had been in talks to open the tour before Minaj got onboard.

Over in the UK, Take That snagged Pet Shop Boys as the opening act on the Progress Live 2011 Tour, set to hit the road in May, despite the fact that the two acts ruled in different decades. (Fun fact: Newly returned Take That member Robbie Williams and PSB collaborated on “She’s Madonna” and “We’re the Pet Shop Boys,” two tracks from the former’s 2006 Rudebox album, and Pet Shop Boy Neil Tennant sang on Williams’ early solo hit “No Regrets.”) Once again, though, the effect will be reciprocal. Pet Shop Boys will attract gays and the ’80s-obsessed, while Take That will pull in gays and children of the ’90s.

Combine and conquer! It’s a concept that for years has worked for annual multi-artist tours like Ozzfest and Lollapalooza, yearly one-off festivals like Coachella and Glastonbury that touch down in the US and UK as well as the ones that regularly land in places like Argentina and Australia, and double-bills featuring reuinted ’80s icons. Fans will hand over the cash if you give them hours of entertainment featuring a smorgasbord of talent. This, however, might be the first time we’ve seen so many superstar acts settling for the opening slot, and it’s likely just the beginning. Can Ke$ha as the appetizer for Spears’s main course in Europe and beyond be far behind?