Is it possible for two (or more) persons to be working on a Form at the same ti

Hey, yes it can be done

There's no problem if 2persons are working on the form on same time but, make sure you don't rollback the changes done by the other person for example if the other person did some changes but the code you were editing is the older one, so then if you'll save the form then the changes done by the other person will be rolled back.

I need to refine my question: This is regarding process - In my application, two persons will collaborate to work on a document - with one person writing/editing and other person just viewing it.

I can't think of a way to accomplish this without at least incorporating a form-submit on the part of the writer/editor.

Once submitted, will they be able to collaborate on changes/updates? One is updating while other is viewing. Thanks.

The person who's editing could continue editing, and the person who's viewing a read-only version could see the last version that was submitted to the process instance via a Submit button.

I don't think there's any more of a "real-time" option than that without some very specialized plug-in.