2. When a combination of two fields is a common filter in the query string, two-column indexes typically help you sort and display records.
For example, for the following SOQL, which appears in pseudo code, a two-column index on f1__c,f2__c is more efficient than single indexes on f1__c and f2__c.
WHERE f1__c = ‘foo’
AND f2__c = ‘bar’
Two-column indexes are subject to the same restrictions as single-column indexes, with one exception. Two-column indexes can have nulls in the second column by default.