The 140-character giant today unveiled its biggest partnership for ad-supported video embedded in tweets — just in time for Premiere Week. Twitter and CBS announced a deal that includes 42 of the company’s programs across 20 brands, with the goal of getting clips in front of as many potential viewers as possible. “Second screen” interaction while watching TV continues to draw broadcasters’ attention; during last night’s Emmycast on CBS, for example, tweeting maxed out at 17,000 per minute as Carrie Underwood belted out the Beatles’ “Yesterday”. And showing ads simultaneously on a TV program and Twitter can help advertisers spread their message. Since signing up ESPN for the vid-clip service in May, Twitter has brought in such showbiz partners as Discovery, MTV and BBC America, but the CBS deal represents by far the most brands and shows so far. It’s Twitter’s latest move as it ramps up for an IPO.