You can have more than one Keystone at a time but the secret is that you've got to keep them stored in your mailbox.
Anytime you get one, don't loot it, let it there on the body/chest (or even the weekly chest on your class hall)...
... and as soon as you change map, you will see it in your mailbox. As you can see in the screenshot, I've 4 of them. And I've had some of those for months now. How? well, if you read the Duration it should say something between 7 days or hours until it's destroyed, but on your mailbox, when the item is expired, it's duration is reset everytime, so you can keep them forever :)
