IIRC, the only double-weapon so far in DDO is the quarterstaff.
And even though it is a double-weapon, it's treated like a greataxe that bludgeons rather than slashes (and has a lower damage die).
Here is what I propose...
A button that can be toggled on and off should be implemented (for example, something similar in design to power attack or combat expertise). If it is turned on, all double-weapons are treated like fighting two-weapon style, so you loose some attack in order to get the chance of extra attacks.