I wish to tap two out of phase 120v circuits and achieve 240v such that I can run an electric stove. I need to know which (either if not both) neutral wire to use. I feel I should remove all other devices and receptacles from the two lines as well. Is this good?

I don't know what you mean by

I wish to tap two out of phase 120v circuits and achieve 240v

There is no neutral in a 240 circuit. In a 120V circuit you have a hot that supplies the power and a neutral which returns the power to it's source.
In a 240V circuit you have two hots which are in opposite halves of the cycle. You cannot rewire a circuit and make part of it 120V and part of it 240V.

My instance would be using existing wiring. I understand that the nuetral returns to the source for the 120v circuit. I would like to know then what is the 3rd and 4th wire on 240 receptacles of 3 prong and 4 prong types. Thanks in advance. Perhaps I have confused phase with cycle.

I'll let Harold address your last question, but I wanted to present the fact that most 120V wires are of #14 or #12 size and are not rated to carry the AMPS usually required of appliances/tools that use 220-240 Volts, so be sure you keep that in mind when trying to fast track this job!

If you tell us exactly what you are trying to do (what tool or appliance you are trying to set up by doing this) we can tell you a lot more, OK?

Mark

Thanks Mark, I have not looked at the existing wiring. I was told that #12 in a short dedicated run might be OK albeit not recommended. I feel that the 'fast track' fix only satisfies the customers budget needs. I am going to tell them to do it right or not at all. Has stated an electric stove/oven for a kitchen.

I don't like the sound of this at all!!

First off, the OP sounds like they have no idea what they are doing, and that they want to take two existing receptacles circuits and combine them for a stove circuit? If so this is incredibly unsafe and incorrect.

twangwe (https://www.askmehelpdesk.com/members/twangwe.html), please describe a little better wehat it is you are trying to do here.

Edit:

Thanks Mark, I have not looked at the existing wiring. I was told that #12 in a short dedicated run might be OK albeit not recommended. I feel that the 'fast track' fix only satisfies the customers budget needs. I am going to tell them to do it right or not at all. Has stated an electric stove/oven for a kitchen.So you are not even the homeowner?
Tell them to hire a qualified electrician to do this job right or not at all.
If you were planning on doing it for them I would highly suggest you refrain from doing electrical work for others.