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By using two separate camshafts, you can do a myriad of things.
1. You can use different grinds- lift, duration, etc. When looking at aftermarket cam specs, the intake and exhaust numbers are usually different, i.e. Jun Stage 2s- intake lift is 12mm, exhaust is 11.5mm.
2. It allows you to individually time the intake and exhaust valves. When tuning, you can advance one while retarding the other, advance and retard different amounts, etc. If you were restricted to the same camshaft in charge of both, what ecer you did to one you would have to do to the other, and in the same amount.
3. The big one- overlap. By using two separate cams, you can time the intake and exhaust valves so that there is a little bit of overlap, or time where both the intake and exhaust valves are open. By doing this, exiting exhaust gases leave a vacuum inside the cylinder, drawing in fresh air fuel mix. In a SOHC, only one side can be open at a time.
Making sense?
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