IPhone Backup

Yes, you can back up multiple devices to iTunes. Be sure your devices have unique names. On the phone: Settings>About>Name.

You can also back up any device that is signed onto the same iCloud account to that iCloud account.

Here are some support articles that should help:

About backups in iCloud and iTunes - Apple Support

Back up and restore your iPhone, iPad, or iPod touch using iCloud or iTunes - Apple Support

Cheers,

GB

The first article lists everything that iTunes back up:

An iTunes backup includes nearly all of your device's data and settings. An iTunes backup doesn't include:

• Content from the iTunes and App Stores, or PDFs downloaded directly to iBooks
  You can back up this content using Transfer Purchases in iTunes.
• Content synced from iTunes, like imported MP3s or CDs, videos, books, and photos
• Photos already stored in the cloud, like My Photo Stream, and iCloud Photo Library
• Touch ID settings
• Apple Pay information and settings
• Health and Keychain data
  To back up Health and Keychain data, you'll need to use Encrypted Backup in iTunes.

I don't know what you are asking in your second question. If both phones are logged into the same iCloud account, then any backup made by either will be available to restore from.

GB