You don't rebuild the other because it is not a regulator (with internal moving parts) like the part you are replacing. The other piston is a pressure bypass fitting that only operates with overpressure. So basically the other piston is just a hunk of plastic that is put into action when pressure overcomes the spring resistance. I'll take a picture of the overpressure piston when I get home.
By the way, if you're rebuilding t-stats? You may want to look at rebuilding the t-stat in the engine. It uses the same regulator. Check out this useful post from burgermeister Frank.
http://forums.pelicanparts.com/911-engine-rebuilding-forum/689055-internal-oil-thermostat-repair.html