Making a 40,000 mAh external battery
Yes, it is possible, but highly inadvisable. Here are a couple of reasons for that:


When manufactured, the cells in the power bank are sorted and matched closely by their internal resistance and voltage, to ensure you get full capacity out of the bank. When cells are closely matched like that there is very little interaction happening between the cells (that would be wasted power), and you get the most efficient use of the cells because they charge and discharge "as one". Mismatched cells from two different banks would waste power internally.
Safety of the bank benefits from the cell matching done by the manufacturer. The most dangerous thing that can happen is overcharging, followed by excessively discharging the individual cell. In a matched bank, the battery management system monitors the voltage level of the cells and usually their temperature to get a grasp on when to stop the charge from flowing in and out of the bank. Most banks do not monitor individual cells in parallel packs, they operate on the assumption of matched cells, which you won't have if you mix two banks.