Pretty simple to dual wield...
Left click: Controls left weapon fire, Right click: Controls right weapon fire
If each weapon had a -secondary fire-.. You could set the middle mouse button or some keyboard button to toggle different modes ie..
Press middle mouse
Toggles left weapon primary right weapon primary
Press it again
Toggles left weapon secondary, right weapon primary
Press it again
Toggles left weapon primary, right weapon secondary
press it again
Toggles left weapon secondary, right weapon secondary
And loops from there.. Obviously there would be a HUD that would allow you to see which weapons functions are toggled to be more easily remembered/viewed/and changed.
WindowsXP Home(Service pack 2), Athlon XP 2400+(2.1Ghz), 1GIG Ram, Ati Radeon 9800Pro 128MB.