Provided the pump is sized for the index radiator when both zones are in operation, there will be no problem. If the zones are running separately there will be a separate index rad for each zone; but when both zones are running the index rad is whichever of the two index rads has the greater head. The flows and losses of the separate zones are not added together. Any combination only occurs in the common pipework, i.e.the 22mm.I have 10 radiators downstairs and 7 radiators upstairs. My worry about only having the one pump would be that it still wasn't big enough to overcome the resistance of the pipework and I would only know once it had been installed.
This diagram may help: