[licker]

I'm pretty sure they will engage the army outside the fort before the storming, I'm not sure if you also ordered your besieged force to break siege if everyone would show up at the same time, though my guess is no, you would have two separate attacks.

best bet is to actually suicide them (or use returning or something to pull them back out, this won't work if its at your capitol though I think) while they spam you a few rounds of something particularly nasty to hopefully damage the besieger enough that his ultimate storming fails.