Most Co-head situations I see are either higher-ups setting up a power struggle between 2 newly appointed co-heads, or an existing head pretending to step back by appointing someone as their co-head who they can sabotage in real-time.
Larry is still the largest single stockholder of Oracle (~27%). So everyone knows he still runs the company at the end of the day.
Benioff only owns ~2% of Salesforce. There's institutional investors who own more of Salesforce than even Marc.
Their first non-German leader (Anshu Jain, who didn't speak German) was made Co-CEO with an older German hand Jürgen Fitschen from the board.
They pushed out Jain after 3 years, brought in a Brit John Cryan (who spoke German!) to co-CEO with Fitschen. Fitschen stayed on another year after which they finally let the non-German, Cryan run the place as sole CEO for 2 years before replacing him with another German as sole CEO.
So in total there were 4 years of non-German co-CEOing with a German, 2 years of a non-German actually being sole CEO, and then back to a German again like the rest of their 153 year history.
It's actually worked quite well at Oracle over the decades.
Benioff, who is former Oracle, hasn't perfected it like Larry has.