I think the rules don't cover it. But I would not allow them to stack. This equip is too good to be too cheap.
The rules of making equips are vague and don't cover well magic items that don't follow a specific spell. Striping the other powers of is the same of making a brand new item, with just the CHA to Attack/damage. The rules don't cover that type of ability, so it would demand for DM to fill this issue with houserules.
Filling in blanks with houserules, a daily happening
Would switching the target attribute from CHA to INT be a problem? I foresee, if possible, a lot of rogue/scout/invisible blade/swashbuckler/ wizard / Swiftblade builds running around sticken people for pretty good damage, given a high att score. ? Would this stack to damage w/ the swashbuckler?