As far as I understand, you might have added validation rule to one of the form fields,but the validation rule uses value of other field for validation. In this case you should use field1.validate(), this will automatically validate field1 against field2.
If you have multiple such field combinations, and are adjacent to each other(to be accomodated inside a panel then wrap them in a panel and use below mentioned rules.
If you are doing this from rule editor, you should preferably use panelName.validate(), which will be better and cleaner approach.
If the fields are not adjacent, use field1.validate();field3.validate();