Hello scenic26,
Welcome to Apple Support Communities. It looks like you'e having issues backing up one of your iPhones using iTunes on your computer. Your number one defense against losing content is to have a backup of your device in either iCloud or iTunes on your computer. This simple step can be so handy, and we'd like to help.
Are you signed in with the same Apple ID on both devices? Also, what is the exact message you're getting when you try to back up your device?
To get started, please checkout these helpful links:
If your computer doesn't recognize your iPhone, iPad, or iPod
How to back up your iPhone, iPad, and iPod touch
Back up your iPhone, iPad, or iPod touch in iTunes on PC
Back up iPhone
We hope this information is useful. Kind regards.