Unable to assign a weapon with attack skill of 9 to weapon set 2

I am a sorceress wielding two staffs. The staff equipped to weapon set 1 is level 9. The staff equipped to weapon set 2 is level 8. I am unable to equip a level 9 staff to weapon set 2. I have proven that this is not a stat or level requirement issue because I cannot even swap posititions for the two current staffs I am wielding (i.e. I can't assign the lvl 9 staff to set 2 and the lvl 8 staff to set 1)