The two had separated briefly when Miller left Uber to join rival Didi's new self-driving lab in California.
Miller announced today that he was leaving Didi after just four months.
Valasek is leaving Uber, where he heads up security technology for the ride-hail player's self-driving cars, sources familiar with the matter told Recode.
Uber confirmed his departure.
"We thank Chris for his contributions to Uber and wish him all the best in his next endeavor," a company spokesperson said in a statement.
The duo joined Uber in August 2015, mere months after the ride-hail company poached a slew of top engineers from Carnegie Mellon University.
Valasek tweeted that he and Miller will be heading up Cruise's security team.
—By Johana Bhuiyan, Re/code.net.