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)