Complex sentence: 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).
Split into simpler sentences: 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).