Generally one spark plug is sufficient to ignite the charge inside the engine cylinder. But inside the engine the charge burns with a particular burning rate. In case the piston is moving too fast inside the cylinder (In high speeds) then chances are developed i.e. charge did not completely burn and the end of working stroke has come.In case combustion starts by the help of two spark plugs at two different locations then the aforesaid problem can be resolved. This video tutorial clearly demonstrates by animations i.e. why dual spark plugs are used in SI engines.