If one was to apply for a new passport before the old one expired, would it be possible to continue to use the old passport?
It being the case that the old passport was not physically marked with the words cancelled, etc, and was not reported as lost/stolen.
I know some countries allow citizens to hold two physical copies of the same passport. So I believe it would be akin to this, but not officially sanctioned.
From the New Zealand government passport application form:
WARNING Once your passport application has been received, your old passport will be cancelled immediately and cannot be used for travel. Anyone attempting to travel on a cancelled passport will be refused travel.
The process is still the same as countries that require the physical passport for renewal: they will mark your old passport as cancelled in the relevant databases when they receive your application, and the document will no longer be valid for travel. Many countries will punch a hole or otherwise mark your old passport and return it to you (so that you may continue to use any visas contained therein, if this practice is accepted by your destination country); New Zealand asks its citizens to take care of the marking themselves to facilitate a more efficient process where the old passport need not be mailed in.
Some countries do issue multiple passports to their citizens, if and when the citizen has a good enough reason (in their opinion).
But using an invalid passport, stamped or not, is a very bad idea. So the question would be, at which point in the renewal process does your government invalidate the old passport? If they instruct you not to use it any more, as you mentioned in the comment, that point has been reached.
The answer is yes in most of the cases! Otherwise, how would you be able to travel in this situation? I mean, if you apply for a new passport, it will take few days/weeks before you get it. So if you can't use the old one, you can't travel.
Indeed, it is better to apply for a new passport before the old one expires. This will allow you to fill the gap and avoid a period without any passport available.
And you are right, some countries allow you to carry two passports at the very same time, mainly because you could denied access in some countries if your passport has visas from other countries because of political reasons...
