After the last update it's possible to make stacks of tools and other items, like air tanks. Since said air tanks got a fix the last time something like this was possible, I assume it's not an intended feature.
With 1-use items it's straightforward:
1. Craft the items one by one.
2. Keep all crafted items in your inventory.
3. Drag one item over the other and release.
4. If it doesn't work at first, just try again.
5. You should now have 2 items using only 1 inventory slot.
* Some items split when put into storage like this, some don't.
* Works with single items and with stacks.
* Naturally, works only on the same kind of item.
* Using such an item will only consume 1, not the whole the stack.
With tools, you have added durability:
1. Stacking works just as with any other item.
2. Dragging both tools with the same durability changes nothing.
3. Dragging a new tool over a broken one will show as 2 broken tools together.
4. Using up a tool like this will leave the other one on the stack as a brand new. Stack shows only the durability of the current tool, doesn't actually damage the others.
5. Dragging a broken tool over a new one will completely restore it's durability. Both will now show as brand new tools, because they basically are.
In short, stacking items not only ignores designed balance of the inventory, but can even lead to unlimited tool durability. After almost using up one tool from the stack (eg. a shovel), when it's in the red, all you need is a new copy inside a storage box. Move it to the inventory and drag the broken one over a new one. Then just split them (shift), leaving just 1 in storage.