I would like to request the option for Waze to display units in both us and metric concurrently. It would be very helpful for learning to switch between the two!
As a step in that direction, just set Waze to use the system that your vehicle does not use 
I think the option of displaying both could be helpful for some (via check boxes ie: display metric, imperial), but would also add what many could view as “trivial” information to a congested interface. I wouldn’t make it a required view.