Composite Secondary Index- - response (3) by j355ga

There is no way to guess which is better -  you will need to experiment. Try both single and two col index.  Review the EXPLAIN plan for both. Compare the performance of no SI, single SI and two col SI and decide which gives the best results.  There is no one definite answer when it comes to picking PI, SI etc.
 A covered query is one in which all of the columns in your SELECT are also in a SI or Join Index.
If you have col a, b, c in your sql but the SI only has col a, b then the optimizer will not consider the SI for a covered query. 
Read this post for more on covered query http://forums.teradata.com/forum/database/covered-query.