If you are playing something normal sized (like 2-3k each), I'd suggest the opposing players split the DD pool.
ie. you roll a 5 and a 2.
7 PD
3 DD def player 1
2 DD def player 2
Might work, just a thought. (1st player clockwise gets the extra DD, if any..)
Another option might be that each defending player get DD equal to the lower number rolled.
But, yeah, there's no real "rules" for multiplayer...