The building system feels un-necessarily complicated. this could easily be fixed by just forcing pickaxes and blocks to take up both hand slots when selected.

Starbound does this in a great way by having every hotbar slot account for both hand slots, allowing you to customize both hands in each slot. When a block is placed in one of the hotbar slots, it takes up both hands so you can left click to the foreground, and right click to the background.

Simple, intuitive. Doesn't require swapping items around your hotbar or keeping one slot empty to achieve the desired functionality.