Here's a USA Today article on Salesforce. Under the second subheading they say,
"Salesforce was born in March 1999 in cramped, adjoining apartments atop San Francisco's picturesque Telegraph Hill. Benioff, still employed at Oracle, lived in a condo. Fellow co-founders Parker Harris, Dave Moellenhoff and Frank Dominguez set up shop next door. Computer cables ran between the apartments through windows. 'It was the Gold Rush era for tech start-ups then,' Harris recalls."
And if you Google on Parker Harris at least, you can find him described as a co-founder.
By the way, SAIC is an interesting one. The main guy founded it with "a handful of scientists" and one page describes someone as a "Vice Co-Founder" of SAIC. "Vice co-founder!" That's harsh.
However, I don't know if it's terribly useful to include ancient companies like IBM, Beckman or Avnet. Although it does help show that the trendline is going up -- Salesforce, one of the most recent, had four co-founders in 1999.