A friend of mine confirmed this last night:
XP stone, white tiger charm, blue dragon charm can all be used at once, for 190% exp gained (at least, that's the assumption, I don't think she monitored the gained exp for differences
). She was quite happy with the results, though.
I couldn't tell you about other combinations of charms.
Also, considering the "Can't gain more than 200% exp" jargon written on the stones, I would hope the exp gains from the charms stack. =/ How're you gonna hit 200%+ otherwise? Stone + dragon charm + tiger charm + phoenix charm's the only way to get that high atm, assuming you can use three charms at once.