I have a script for your second hand
It adds 5 extra slots (at the left of the other slots), but they are for your left hand.
It's too buggy to use:
1. If you have a dual weapon, Blockland crashes
2. The weapon in your left hand will only shoot/do something if you press another button, not the regular fire button
3. Uses another playertype, not the standard player
4. Many more bugs...
5. ....
It's too buggy to release, so I'll stick with it