Depends on what you are cleaning. Nobody makes a dual pumper that actually holds up or works for a long period of time without unloader problems or something going wrong. A bare minimum would be 3000 psi and 16gpm.
To clean efficiently you need a minimum of 4gpm per gun thus your minimum total gpm will be 8. The pressure required will vary depending on the type of cleaning.
Cleaning Units per Hour = GPM x PSI x 60 MINUTES
EXAMPLES: 4 x 3000 x 60 = 720,000 CUPH
5 x 3500 x 60 = 1,050,000 CUPH