When you have several charms in your inventory, you can put them all onto a one-slotted weapon or armor and get the effects of all of them. Go to your weapon display, click on the charmable weapon so that it brings up the plate with the charm slots, then keep clicking on the weapon to bring up several charm slot plates. Once you have enough, drop one charm into each plate and close them as soon as you finish. You can put nearly unlimited charms on this way, however open all the plates before you start charming. Any that you open afterwards will have the charms already inserted. Also, this can only be done once per charmable weapon/armor, assuming that you exceed the available charm slots on the equipment.