Progress uses single index when using Find statement or CAN-FIND statements. Progress uses mutliple indexes in for statement, when progress uses multiple indexes the time reduces since multiple indexes are used, but will it decrease the number of DB reads ? Also is there any documentation or reference containing details how progress selects multiple indexes and whether it is effective or not ?
Have a look at the ABL Triggers and Indexes manual in the documentation for your release.
Also, for verification of index selection, look into: