Kristen Stewart and Robert Pattinson attend the Teen Choice Awards on July 22, 2012 in Universal City, California. Credit: Kevin Mazur/TCA 2012/

If only it had been a double date!

On Saturday, Robert Pattinson and Kristen Stewart made an extremely low-key appearance at Chateau Marmont in Hollywood -- the same evening that Jennifer Aniston and fiance Justin Theroux were spotted having a romantic date at the famed hotspot.

Two witnesses confirm to Us Weekly that Pattinson, 26, and on-again girlfriend Stewart, 22, arrived together at the club for the birthday party of a pal. Recently reconciled (and living together again) following Stewart's July fling with Rupert Sanders, the couple "snuck into the garden from a private side entrance," according to one observer.

Another witness adds that Pattison was wearing one of his many baseball caps (like the one Stewart was snapped wearing during her fling with Sanders, 41) backwards. During the evening, the couple (casually dressed in T-shirts and jeans) were "whispering very close and intimately," the second witness says.

The reconciled Breaking Dawn Part 2 costars' outing together was on-the-downlow by design, of course.

"They're afraid of backlash, so they're keeping things on the down-low," a pal explains.