The chance to find more keys after the first one is greatly reduced (unless you use a minor exploit, abusing the window to loot corpses) but you can use as many keys as you have on you.
This has been something I have been wanting to make a video of for quite awhile. Here is how I have been getting more than one Key per week per character since the Isle of Thunder was released. Was discovered by accident, and Blizzard has never acknowledged it (exploit or not).
Please see the description for more information.
You can use items from TI to speed up the travel between rares, too (the one that increase mount speed and the one that decrease the aggro radius).