You can get another FP after the earliest return time on your current FP. If you get a FP at 12 and your return time is 1:10-2:10, you can get another FP after 1:10, whether or not you have used the first one. So you could have two at once. Disney has no way of knowing when you actually use them, just when you get them.
jaysmom is right about if the time is close to when you got the FP - the 2 hr rule applies to longer stretches of time. Say you get a FP at 12pm for 4pm.
The new fastpass system tells you on the bottom of the last FP ticket you got what time you may get your next ticket. More Fastpass info: http://www.mgmstudios.org/fastpass.html
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