If one piston has less stictiob than another it’s not uncommon for it to move first. Note you’ll only notice this really with the caliper off the disc. With the caliper on the disc the piston that moves first can only move so far before the pad starts to bind on the disc, at which point the other piston is freer to move and that too comes out until the same point. In short, I wouldn’t worry if one piston moves before the other. The piston’s retract due to a combination of the shape of the rubber lips of the seals within the piston and fluid mechanics once the pressure is released. They only need to retract a fraction of a millimetre for the friction between pad & disc to disappear. The grease you can use on the pistons, and should use on the caliper seals if they’ve been disassembled completely for service is a special brake grease such as ATE Bremszylinder Paste.
