Representatives Maxine Waters of California and Joyce Beatty of Ohio included the two big private equity players among a list of 31 investment managers who received the letter.
California's second-biggest public pension has been growing its co-investment programme steadily for several years as part of its system-wide 'Collaborative Model.'
Wayne Moore also asked that staff and JP Morgan, which runs the private equity emerging manager programme, provide more information about the demographic make-up of its GPs.
Some board members have voiced concerns about the pace at which the $20bn pension is implementing plans to branch out into secondaries and co-investments.
Staff had predicted a major comeback for the $80bn pension's PE programme at previous meetings, saying relative underperformance earlier this year was due to a 'benchmarking anomaly'.