How to validate several fields of a registration form

Good morning.
I want to do a combined validation between two or more fields, which already exist in a Registration form. How can I do it?
It does not have to be at the moment, it could be done at the end of the member's registration.

Thanks

Combined validation? Can you be more specific about what you're wanting to do? You want to combine the values of 2 fields then validate that?

OK, Krileon.
I have two fields of guarantor Membership number on the registration screen.
Field 1: Guarantor Member Number 1. Validates that the member exists and is over 18 years old (with CB Query Field).
Field 2: Guarantor Member Number 2. Validates that the member exists and is over 18 years old (with CB Query Field).
Now, what I need is to validate that the guarantor specified in Field 1 and Field 2 live at different addresses. 

How can I do?

You should be able to do that with CB Code Field and its Code Validation. You'd then just use PHP and compare the two fields values. Which field you add this validation to is up to you (could also just add it to both). Example as follows.

If these fields aren't marked required be sure you take into account them being empty. This will also work with ajax validation for client side validation if you adjust the Additional Fields to include the other address field.