As I recall they stack, and their penalties and bonuses don't stack, but you take the worst. So if you had a drawback -2 to defence and -3 to defence, then you take -3. And if you are taking -2 and then take lose your defence, then your defence is zero. And if you lose your defence from two different manoeuvres, then you can still do both, and your defence is still zero.
Is this right? Something I read seems to indicate otherwise.
One of my players is combining
Spetsnaz Knife Fighting,
Two Weapon Fighting (main gauche + rapier) and I'm seeing some rather crazy things, like 17 defence (by declaring dodge). and a free attack in retaliation each round.