Yes. That is the correct interpretation of the Max Head capacity of a pump. Note in your theoretical example though, the water pressure and flow rate would be around zero at the discharge point (because the max head of the pump is 20 metres and the sum requirement of your example is also 20 metres). In practice, if you're pumping requirement totaled 20 metres, you would want to get a pump with a max head capacity of greater than 20 metres, to have sufficient water flow and account for small head loss from friction.