I run 40-50psi, whether I ride one up or two, but then I am in the security size squad. I thin if you are an average size rider you would probably be happy in the 25-35 range for most of your riding, unless you go for a little stiffer suspension then pump it up a bit more.
I always found that when the shock was under 20psi the rear end was just too squirrely. Just watch you don't over pressurize, if I remember correctly 50psi is max.