well, let's see...

2:1

4 attacks, each x5.

(20+20)*4=160, as opposed to 80 for 1-handing, thus makes the difference 80.

80x5=400

182.5x3= 527.5, does it not?

Of course, two-handing, I'm sure, would be much more reliable (considering all the minuses of two-weapon fighting)