Answer for USA, Canada and countries running a 60 Hz supply service.
This is not a good idea. It seems logical but I'll explain why it's not. Each circuit is set up to feed a given amount of amperage. This is why there are so many wire sizes, because it depends on how much juice is going to be forced through them. Now a neutral is actually where the leftovers go, so combining two together to go into one wire before it hits the neutral bar in the electrical box will probably cause that wire to overheat, which breaks down the wire and eventually bad things can happen.
It's always best to put a 240 volt appliance on its own circuit. (Make sure you have the proper wire size for what you are trying to hook up.) If space in your breaker box is a problem, and you can't spare the cash to get and install a new one, you have the option of getting a small single or double box and attaching it to your old one to achieve more room. Remember, it's better to be safe than sorry.
No. A 240 volt appliance should normally have its own dedicated branch circuit and circuit breaker and must be arranged so that both poles are simultaneously disconnected. (One handle or two handles that are mechanically connected.)
As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.