Double camshafts are not required in order to have multiple inlet or exhaust valves, but are necessary for more than 2 valves that are directly actuated (though still usually via tappets).
Double camshafts are not required in order. This was to have multiple inlet or exhaust valves. Double camshafts are necessary for more than 2 valves (though still usually via tappets). These more than 2 valves are directly actuated.