I have a two fields in Tax master object Excise_Duty__c and Sales_Tax__c how to write a validation for Only one of the two to be entered. i am wrote a formula like this:OR(NOT( ISBLANK( Excise_Duty__c ) ),ISBLANK( Sales_Tax__c ) )This is working for only one field for checking condition,how can i check for both fields!Please provoide me profer solution for this!! Thanks in advance