There is no "proper way." If by "one hand one way and the other hand the other way," you mean one hand supinated (under) and the other pronated (over), AKA, a mixed grip, that is fine, provided you switch hands regularly. That is, switch which is over and which is under, either every set or workout.
Both pronated is good too, but grip strength is a limiting factor. It's just easier to hold on with a mixed grip. If you have the strength to hang on with both pronated--go for it! Plus you have the bonus of not having to worry about switching and it evenly distributes the load. Greater stress is placed on the biceps muscle of the supinated hand in a mixed grip. Not a huge deal, but it can lead to a biceps tear in some cases if you're not careful (or even if you are careful).