You can, but for most people, the best solution is to share an ID for purchasing from the iTunes and App stores, but keep separate IDs for iCloud, iMessage and Facetime.
If you share the same ID for iCloud, you will be sharing the same iCloud account. When you do this, any data you sync with the account is merged and appears on both phones. You will, for example, end up with each other's contacts on your phone. You will also be sharing the same 5GB of free storage space that comes with each account vs. each having your own 5GB of storage. You can still share your iCloud calendars if you want (http://help.apple.com/icloud/#mm6b1a9479), and while you won't be sharing the same photo stream, you can still share photos with each other using shared photo streams (http://help.apple.com/icloud/#mmc0cd7e99).
If you use the same ID on both phones for iMessage and FaceTime, you'll end up getting each other's text messages and FaceTime calls.
This article may be of interest: http://www.macstories.net/stories/ios-5-icloud-tips-sharing-an-apple-id-with-you r-family/.