Simpson was on a date with boyfriend Eric Johnson, while Lachey was with longtime girlfriend Vanessa Minnillo when the foursome all showed up for dinner at Hollywood hotspot The Red O.
Simpson, 30, and Lachey, 36, ended their marriage five years ago, but the former Newlyweds stars' encounter was surprisingly drama-free.
"They didn't interact," says a source. "They didn't even come to face-to-face with each other."
"Jessica was more fazed by the amount of paparazzi than anything else," adds the source.
In fact, another source tells PEOPLE that while Lachey and Simpson are are no longer in touch, their relationship remains amicable.
As for Lachey, he and Minnillo are going strong as ever. "They're very happy," says the second source. "He wishes [Jessica] the best."
Last week, Simpson told PEOPLE she's in love with Johnson, and that "if right now could last forever, I'd take it!"