When you take the last mastery level in Twin blade, you do not get the improved two hand fighting feat that suppose to give a extra off hand attack with penality.
It seem to work fine for Saberstaff.
I also tried to refund the saberstaff mastery perk to test if it was a conflict between the two.
Did you try unequipping other gear and reequipping?
Ofcourse i did , This is been issue for a loooong time but since it dont seem to get fixed i did some troubleshooting and the main problem is you do not get two hand fighting feat when twin blade is equiped and you do with saberstaff.
Maybe a typo in the script on Twin blade
This is a bug I’ve known about for a while, but thank you for submitting a report about it here so there’s a reminder about it somewhere 