One employee wants to work two jobs: How do we calculate overtime?

Q. We have a hourly shop employee who works 40 hours plus overtime per week. This same employee also has his CDL (commercial driver's license) and would also like to help out after his shift in the shop by driving our truck. How would we pay him since it is a totally different job?  Would we have to pay him overtime or would be able to pay him a set pay per each trip, or could we pay him a different hourly rate for that driving job? – Mary, Nebraska