for each user, get a new and unique e mail address. google is free, and a fine place to do that. In settings, facetime for each device, add that unique address as a new 'receive at ' address. note, when you add the address, apple will send an e mail to that new address to verify that it exists and that it is yours. The new address wont stick until you respond to that e mail from apple, so be sure to look for it.
This way each device will have its own face time address.
The same thing will apply to i message, if you want each device to get its own messages.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
iOS: About Messages
Set up iMessage