Unfortunately, I think there is a bug where the game will combine 2 different penalties.
So having 2 penalties on the arms (20-29 encumbrance) will result in expected results, but having ‘uneven’ penalties will combine the two penalties.
So having the right arm at 17 encumbrance and the left at 24 encumbrance will create a total penalty of 5.
Theyre trying to fix it but I can only say its worth it to test it yourself to see if you are okay with the resulting setup.
Use a shield and see. debug one in and go slaughtering. Or, like me, stick to using arm guards.