Antoine - Are you pulling a train with these two locos or are they just running light? If they are running light (two locos and no cars behind them) then oftentimes they will not run perfectly smooth. If you are pulling a train, especially a heavy one, then the load sharing capability actually has something to share. The locomotives will run much better pulling a load. Without a load there isn't much for the MU/load-sharing function to go off of, and you can get that "bucking" you described. It can be more prevalent around 18% power, then smooths off once you are in the 20's. Why it occurs more at that point I'm unsure of. I think at that point the load-sharing switches from a low-speed function to a high-speed fucntion and at 18% the transition can be a bit rough with certain locomotives.
I have a couple locos that will buck at 18--20% when they are not pulling a train. After that they are fine. With a train behind them they run perfectly at all speeds.
Regarding the updating of modules....check the module's software version by tapping the info button on the locomotive screen (the one that takes you to the real-time info page with track voltage, temp, etc). At the bottom of the page it will tell you the software version currently installed on the module. When you update the software on a module the screen will say "Programmer Started" and show an exit button at the bottom. "Programmer Started" means the update was successful. Exit the page and check the version on the module to confirm. Updating the software version is usually very quick.
-Kevin