>> I have two field. Field A and field B. I want to validate field B when A is changed and vice versa. So, how to trigger validation of particular form field when one field is changed?
You have to make a validation rule for field B using field A and vice versa. We currently have support for revalidation of whole form, not individual fields. This you can configure with help of visual editor.
>> Validate rule applied when field is focus out. As a result, on click submit without focus out from a field, first field is validated and does not submit the form. On second click on submit it submits the form. How to handle this situation?
What is your AEM version? This works on AEM 6.3, i.e, i don't require a second click on submit.