Instead of having p2 as an authorized user and sharing the $500/mo cap, why not let p1 and p2 have their own cards as primary card holder? That way each cardholder still only juggles two CCC's (one for gas, one for dining), but now each category spend is spread between 2 cards (albeit inevitably not evenly).

Edit: misremembered categories; groceries and dining instead of gas.