You can sync as many devices as you want to the same iTunes account, there is no limit. You can have the same content on each device or different content, the choice is yours, iTunes will keep everything straight, by device, upon subsequent syncs. It is easier for you to keep everything straight if you give each device its own unique name I.E.-"Sam's iPhone", "Sam's iPhone Two". That way, you can select the proper backup if you need/want to restore. The first thing done when you sync is an iPhone backup. Only one normal backup is kept by iTunes for each device. It's actually an incremental backup in that only the content that has actually changed is backed up.
About iPhone backups, how to restore from backup, what is backed up:
http://support.apple.com/kb/ht1766