There is an invisible soft cool down on 3ofC and using multiple at one time is a total waste after the first one. The cool down was noted in the weekly update when they launched the hotfix in September stating the coins would be less effective when used in rapid succession but would behave normally when more spread out. Effectively an invisible cool down period. It hasn't been specified by Bungie, but the general consensus seems to be 10 minutes. As for stacking, all that the coins do is activate the "exotic particles" buff on your character which is visible on the character screen. Using one when the buff is already active does nothing. What it means when it says they stack in the description is that, when u use one and get the buff, if the next ultra u kill does not drop an exotic, the next time u activate the buff will have a higher chance to drop an exotic. People figured this out the first week of using them.