I recently rebuilt a spare set of front calipers with new pistons and seals. I bench tested them before installing them. noticing when bench testing that the opposing pistons do not travel out equally and it varied between sides. Granted that only on side is nearest to the brake oil line feed, but that did not make a difference. maybe i had air in the line and it did not give me equal pressure on both pistons. Is this by design or just a pressure issue?
Once they are installed and bled they should be fine. Both pistons are fed from a common source and there will be minute differences in their tolerances - the fluid will take the path of least resistance. When the brakes are applied the pressure exerted on the disc will equalise immediately provided that both Pistons move freely.