IF(
AND(NOT(ISNULL( Assigned_On__c)), NOT(ISNULL(Due_Date_to_Partner__c))),  Assigned_On__c -     Due_Date_to_Partner__c, 0
)
IF
AND(NOT(ISNULL( Assigned_On__c)), NOT(ISNULL(Due_Date_to_Customer__c))),  Assigned_On__c - Due_Date_to_Customer__c,0
)

IF(
    AND(NOT(ISNULL( Assigned_On__c)), NOT(ISNULL(Due_Date_to_Partner__c))),  Assigned_On__c -     Due_Date_to_Partner__c,
    IF
        AND(NOT(ISNULL( Assigned_On__c)), NOT(ISNULL(Due_Date_to_Customer__c))),  Assigned_On__c - Due_Date_to_Customer__c,0
    )
)