[summerlight]

My suspicion here is simple; they couldn't find a proper candidate who can demonstrates technological and operational leadership from both insiders and outsiders. You can simply promote either of them (usually COO suits better though), but that risks departure of another candidate, so this compromise has to be made. Anyway, this might be okay for Waymo for a short term; unlike other companies, even if those two CEO don't agree on a specific matter, there's an escalation path to Sundar (and ultimately Larry and Sergey).