Hi, I presume your system is similar to the Giss Diana.
0 If so, try and keep the backlash between the idler gears and the racks they mate with on the pistons to an absolute minimum or you will have a hard time getting both pistons to arrive simultaneously. To get an idea of this, use a very weak spring to load both pistons with the gears in place, then insert a rod through the transfer port and hold it against the piston seal while increasing the pressure inwards against the rear piston. If you can see it move before you feel the front piston move backwards, it means you have substantial backlash in the gears or their pinions or both.it all adds up.
I presume the dummy piston is bored, you don't want it to compress air at all, you just need it's mass and dimension.

If the front piston arrives before the rear one, you will get recoil and reduced efficiency due to moving a useless rear piston. If it's the other way round and the rear gets there first, you get lost volume and inefficiency. In both cases the gears will always be loaded trying to halt a piston that wants to move some more and that's bad news because they will break up eventually.
Think of the gears as idler gears, they should never be loaded except when the gun is cocked or to a lesser degree when the gun is assembled with the end block off.