how can I back up a second iphone on the same computer?

Unless you are backing up to iCloud, it is automatically backed up when you sync. iTunes will read the serial number and keep the two backups separate. You can confirm that both backups are there by going to iTunes>Preferences>Devices and looking at the Backups list. (Hover your mouse over the backup to reveal the phone number if you're having trouble determining which is which.)

If you are interested in options for syncing multiple devices to the same computer, see http://support.apple.com/kb/HT1495.

Connect iPhone to computer, start iTunes, right click device in device pane, select backup. The backup will reflect the name of the device. Make sure that automatic syncing is turned off before you connect.