Multiple iPhone Backups

The same way you back up one phone. Connect it, click on it’s icon in iTunes and choose Backup. With 2 or more iOS devices when you click on the phone icon you will see a list of phones to choose from. Just make sure the phones have different names, so you can tell the backups apart.

Same as with one phone. The macbook will keep separate backups for the two phones. See the screenshot of the backups on my mac...