Is there any chance the reason they retard the timing is because there are 2 spark plugs???

This feature of an extra plug probably helps build more torque, by getting such a quick burn rate from the 2 plugs it may not be necessary to advance the timing, also it is putting less work on the piston because of the two plugs firing at different times during stroke and the ability to burn so much more efficiently, you might be creating more problems by advancing your timing and not make anymore power.