If I am traveling for one client and work on another client’s file while on the plane may I bill both clients for that time?

No. This is referred to as “double billing” and is tantamount to stealing. If you use part of the travel time to do another client’s work, subtract that amount of time from the travel time billed to the first client. In other words, partition your time and charge each client for the time actually spent working on their file or while traveling in connection with business.