Assign direct shortcuts to items in the second inventory row.
Keyboard & Mouse settings contains eight Enhanced Hotbar slots: Row 2 Slot bindings. They start unassigned. Set one to use that column of inventory row 2 directly, regardless of which row the top hotbar is showing. A separate HUD slot appears when both a binding and an item are present. Modifier combinations such as Shift plus a number can give the second row its own controls.