Lisa

You can see videos of guys shifting both boxes simultaneous -- that is fine for someone with experience. For a beginner it just may be a good way to tear up a gearbox.

Try it a bit simpler. Shift only one box at a time. Unloaded you probably will not need the brownie and probably not on hills.

For initial learning I think the Brownie in low is a bit better. Shift just the main for a while until you get comfy with that.

Mistakes ie missed shift are easier to correct in this manner. Do not use the clutch after you get rolling. Shift only using two or three fingers - do NOT jam or force gears. It will easily slide into mesh when the teeth of each gear are spinning at same RPM. If you miss a shift it will take some tinkering (hunt n peck) to find the new gear match.

Then you can get complicated and split shift ie
MAIN BROWNIE
low low
low direct
low over
second under
second direct -- and so on and so on.

When you get really good then you can do it like shown in the videos.