A CONCERT with heavy metal, alternative rock, and rap all under the same roof? Unlikely as it may seem, this summer's big concert trend has been toward togetherness - genres are mixing all over the country.Some of the packages on the road this summer are Gathering of the Tribes II, a nine-act marathon featuring, among others, the black rock band Fishbone, San Francisco-based white funk band Primus, Christian pop-metal King's X, and female rapper Yo-Yo. British gloom rockers Sisters of Mercy are heading up a concert series in support of their new album, "Vision Thing," with radical rappers Public Enemy (who release their new album "Apocalypse '91: The Enemy Strikes Back" this month). Also included are Gang of Four; metal band Warrior Soul; and Young Black Teenagers, a white (!) rap group. The Lollapalooza Festival that played outside Boston last weekend - one of 20 sites - provided an issue-oriented program. The lineup boasted bands ranging from alternative rockers Jane's Addiction, black rockers Living Colour, and L.A. rapper Ice-T. Groups such as Rock The Vote, Greenpeace, the League of Women Voters, the National Abortion Rights Action League, Refuse & Resist!, Handgun Control, etc. were expected to set up booths, but because of rain, not as many showed up. So what's the deal? Since when do heavy metal headbangers want to hear a bunch of rappers? Have gothic gloomies suddenly decided they like guitar-driven hard rock? Are genre-bending megaconcerts really helping to bring these diverse audiences together this summer? The box- office count at the end of the season will tell the story, of course, but there's more to the trend than a limp economy. Andrew Eldritch, lead singer of Sisters of Mercy, finds nothing strange at all about this potpourri of performers. "I like it," he said in a phone interview. "From a European point of view, it's a much more normal thing. It's sad that it takes a recession in America for promoters to start thinking of these packages." Eldritch pointed out that when his band, originally from northern England, started playing in London, they were always put on the same bill with a reggae band. He recognizes, though, that th ings don't often work that way in the US. "The radio, for instance, is formatted. Even in magazines, if you want to read about a rap band you've got to be on a different page from the rock band page." Eldritch would like to break down the barriers. And he's convinced that there's a racial element involved. "There were certain towns, certain cities in America on this tour where we couldn't find a house manager who was prepared to put both bands on in the same venue on the same night," he said. "We couldn't find a single venue in Detroit that would put both bands on on the same night. This is the city that gave birth to Motown, but the hall would not let us play with Public Enemy." There have been some fears - due to isolated violence at a few rap concerts - that putting a black rap group together with a white rock or alternative band could cause a clash. At the "Vision Thing" concert here at New York's Radio City Music Hall I had a chat with some of the ushers, all young black men. They said that the hall had beefed up security, expecting trouble between Public Enemy and Sisters of Mercy fans. But there was no trouble, and, in fact, a good number of Public Enemy fans, sporting T-s hirts to that effect, were white. "We hope this concert will convince the hall to book more black acts," said one usher. It seemed to be less a question of race, and more of taste. While Public Enemy's high-energy set seemed to appeal to everyone, many blacks walked out when Sisters of Mercy started to play. I spoke with Henry Rollins on the phone before the Lollapalooza tour. He and his rock band, the Rollins Band, are among the many featured groups on that tour. He's all for mixing genres. "Why not? It's nice to have a couple of new colors on your palette," he said. Nevertheless he admits it's hard to get audiences to listen to something new or unfamiliar. "Young people tend to be close-minded; you're into your opinion, your stance, you've got your T-shirt on, so you can't possibly be into that guy's T-shirt. So maybe what we're supposed to do is open some people up," he says. It may actually be happening. I spoke with a few people at the Radio City concert about why they had come, who they had come to see, etc. One young white guy in a metal-studded leather jacket said he'd come to see Gang of Four, but had missed their set, so he stayed for Public Enemy. "They were pretty good," he said. "It was the first time I'd ever seen them. I liked the way that guy brought his kids up on stage." (Flavor Flav, Chuck D's comical sidekick, brought his three very small children out at the end of the set.) Two girls in their early 20s, also white, said they were there for Gang of Four and Public Enemy - would they stay for Sisters of Mercy? "Sure," they said. But attendance has been a problem at some of the concerts. Gathering of the Tribes II's Costa Mesa, Calif. date was described by Billboard magazine as a "stupefying affair" that was poorly attended, with 4,200 people in a 19,000 capacity venue. It would be a shame if the box office take turns out to be the only barometer of success of mixed-genre concert tours.