I almost always give the bracers to Imoen to make her attacks better since Kivan doesn't need them to get the job done. Why have one superb and one mediocre archer when you can have two excellent ones...
There will be times when your entire party may wish to use ranged weapons for at least the opening gambit of larger battles or when charging in blades drawn may be problematic (some areas where there be spiders and ettercaps or even wyverns come immediately to mind)I would probably give the composite to Kivan (assuming he has the best thaco already)and the other to either Sharteel or yourself to use whenever its called for.