Although that would be one heavy engine,it would also be a great accomplishment if completed.I think he is referring to [build a twin] meaning an airplane with 2 single engines,one on each wing.
Since hes running twin ryobis on one right now.
In order to do it youd have to have 2 engines side by side.Both of these engines would need a gear on them and then a center shaft with a gear on it.
That way the right hand engine[turning ccw] would turn the center gear [cw],wich in turn would turn the left engine gear [ccw,as normal].Might have to make a drawing of that sentence.