"There have always been Hollywood couples who do this," says Daphne Brogdon, host of The Fashion Team, a weekly series about celebrity style on the TV Guide Network. "It's a branding thing. They're saying 'We're the glamorous couple' or 'We're the sporty couple.' Brad Pitt and Angelina Jolie do the 'We're chic, we're sexy, we're saving the world' thing. Justin and Jessica do the 'We're in love' resortwear."

Timberlake has that effect on women. He—like J.Lo (see Ben Affleck circa 2001, looking uncomfortable in a pinstripe suit and a Rolex) and Demi Moore (see Ashton Kutcher, often dressed to complement his wife's outfits as perfectly as her jewelry)—is an alpha dresser. When Timberlake and Cameron Diaz were dating, Diaz was seen more than once trussed up in out-of-character plaid golfwear and preppy meet-Mrs.-Timberlake outfits. When J.T. and Britney Spears were a unit, the pop princess coordinated her outfits with his then-flashier, Nashville-fabulous aesthetic. There were strapless gowns with sweetheart necklines and rhinestone chokers. Post-breakup, Spears reverted to her frayed cutoffs and wifebeaters—clearly her natural state.

"I think a lot of times the Hollywood machine will go to work to make two people look like a more compatible couple," says Stacy London, cohost of What Not to Wear on TLC. "There are stylists and publicists and managers and agents all working behind the scenes to create an image. The way a couple looks together is a very important part of who they are as celebrities."

If that's the case, Joe Simpson must be tearing his highlighted forelock out over his daughters' recent makeovers. All that image-crafting—the lip-plumping, the fake tanning, the hair-frosting—was tossed out the window when the Simpson sisters met a couple of rocker guys. Jessica adopted John Mayer's baggy jeans, his shapeless black T-shirts, and even, alarmingly, his dark hair and sunglasses-as-headband look. Formerly Disneyfied Ashlee became barely distinguishable from Fall Out Boy frontman Pete Wentz. Same fedora. Same lank hair. Same low-slung jeans. Same black eyeliner. Jessica Alba? All dolled up, she could compete with Salma Hayek for a spot at the top of a tabloid best-dressed list. But when she was with Cash Warren, she was more often photographed in baseball caps and sweatpants that matched his. And if memory serves, vintagewear poster girl Kirsten Dunst didn't favor Wayfarers, torn T-shirts, and black bomber jackets before she hooked up with British band guy Johnny Borrell.

"Sometimes couples have been together for years and years and they just start choosing the same color palette," London says. (Or, if you're Calista Flockhart and Harrison Ford, the exact same black T-shirt and high-cut jeans.) But the celebrity version of twin syndrome is far more severe than the one that afflicts non-famous couples. It's more like the one that strikes third-grade girls.

"When someone's style changes really quickly in a new relationship, that sort of gives you pause," London says. "And the more they look alike, the creepier it gets. You don't want to think of celebrity couples as related—like brother and sister."

No. You don't. You want to think of them as style icons. They're not political figures—in that arena, it makes sense that pantsuits and ties are coordinated. As professional part-time eye candy, actors and musicians have an unofficial obligation to provide us with more to look at and critique than matching hoodies. The Beckhams and the Cruises are an exception. They may be coordinated down to their frosted tips and Kennedy-style khakis, respectively, but at least they're making performance art out of it. And in the end, some romantic makeovers have their benefits. Maybe that Brazilian model who supposedly got Matthew McConaughey to move out of his trailer will finally get him to put a shirt on.