That would be pretty hard. I don't think there is a mod for this, so the only thing you could do is both make parties of three characters at most and then just attack each other. Only problem is that area effect partyfriendly spells won't work (since everybody's in the same party).
All that is gold does not glitter,
Not all those who wander are lost,
The old that is strong does not wither,
Deep roots are not reached by the frost.
From the ashes a fire shall be woken,
A light from the shadows shall spring,
Renewed shall be the blade that was broken,
The crownless again shall be king.