Can I copy an app from an iPad to an iPad2? I bought an iPad and bought several apps, then I bought an iPad 2. 

Can I copy the apps I bought to the iPad 2 and keep them on my iPad or do I have to buy them again?
This is what the official Apple documentation says:

Your Apple ID can have up to 10 devices and computers (combined)
associated with it. Each computer must also be authorized using the
same Apple ID. Once a device or computer is associated with your Apple
ID, you cannot associate that device or computer with another Apple ID
for 90 days. You can view which devices or computers are currently
associated, remove unused devices or computers, and see how long
before they can be associated with a different Apple ID from the
Account Information page in iTunes on your computer.
Last Modified: November 14, 2011