You can do anything you want if its your building, but it is not best practice.
It is done a lot over cut shingles, but split shakes are mush thicker ond less uniform, so I do not think I would do it if it were mine.
__________________ www.roseroofing.net/ Free inspection within 12 miles
Seamless Polyurea Roofing Membranes for Metal Roofing, Roof Leak Repair, Seamless Commercial Spray Foam Insulation, Seamless Commercial Roofing, SPF Roofing, Industrial Insulation, Our trucks are red but our roofing is GREEN!
If the shakes are laying flat, and if you use long woodgrip screws, it can be done, but be aware that if you walk on the steel, even 26 ga., you can dent it from the back side and it will be visable from the ground. As the above posts advise, it is always better to strip a roof, but we have installed many over shakes with no problems.