You can activate multiple Charms of the same type as long as their effects match. When you activate a Charm of the same type and effect as another active Charm, it simply adds time to the already active Charm.

You cannot activate similar Charms that have different effects. For example, you can activate two 2x XP Charms at one time (even if they have different length timers), but you cannot activate a 2x XP and a 3x XP Charm at the same time.