How do you unit-making veterans make two-handed attacks?
It is possible to make two-handed attacks in Poser without manually adjusting one hand in every frame. You said that the weapon is parented to the right hand, and the right hand controls the movement of the weapon. Now you need to find a way to parent the left hand to the weapon, right?
If I understood correctly and can remember correctly, this is entirely possible. I have actually done it before, at least one time for my Elf Spearman. Here's what I would do, if I were you.
1. Go to the frame where the unit first places both hands on the weapon. I would imagine this is somewhere in the fortify animation.
2. Position the left hand where you want it to be on the weapon.
3. Open the key frame editor, and make all frames, up to and including that frame, key frames. It's probably best if you make all the body parts key frames, especially the left hand, shoulder, and forearm.
4. Close the key frame editor.
5. Turn on the I-K Chain for the left arm.
6. Parent the left hand to the weapon.
7. The left arm (hand, forearm, and shoulder) will move with the weapon.
I just opened Poser for the first time in months, and this did work. You'll still have to do some editing of the left shoulder and forearm though. They won't move naturally as the left hand moves with the weapon. You'll probably need to do at least some bending and rotating. Good luck.