ONCE IS NOT ENOUGH?

What started last fall as a silly soft-core sex ''documentary'' is threatening to become a genuine phenomenon. Last September's Rock Video Girls special was so successful on pay-per-view cable (100,000 households at $5.95 per order) and on videocassette (25,000 units sold) that PolyGram Video has come up with Rock Video Girls 2. The sequel will appear on pay-per-view Feb. 27 and 28 (an active week for PPV; see page 41) and again in March, also at $5.95. Like the original, RVG2 profiles the women who strut their stuff in MTV's raciest videos. Between clips of their slinkiest performances, the women answer questions about their personal turn-ons, turnoffs, and most embarrassing moments on rock-video sets. Among 2's highlights: An interview with Patricia Matthews, sister of former Prince protege Vanity, who's following in big sib's stiletto heels with videos for Christopher Williams, Lionel Richie, and Metal Church. ''My sister and I are very different,'' says the 26-year-old performer. ''She's sexy and gorgeous. I'm more of the jock type. I'm the type guys like to go bicycling with.'' RVG2 hits video stores on March 17. Another PPV special and video are planned for summer, along with a 1993 RVG calendar. ''There's a real market for this stuff,'' says William Sondheim, PolyGram Video's vice president-sales. ''We've even been approached to do Rock Video Girl jeans.'' What? No Rock Video Girl action figures?