Q: When an Apple ID is shared within a home, can more than one device be backed up (on the computer) using itunes without data/contacts/apps merging?

Hi 2confuzing4me,

If you want to keep your calendars, contacts and other data separate, you need an Apple ID for each individual. Here's some information to explain how it all works.

Frequently asked questions about Apple ID - Apple Support
https://support.apple.com/en-us/HT204161​​
 

Create and start using an Apple ID - Apple Support
https://support.apple.com/en-us/ht203993​

Your Apple ID is the personal account you use to access all Apple services. With a single Apple ID and password you can shop the iTunes & App Stores, set up iCloud on all your devices, communicate with friends and family using iMessage and FaceTime, make a reservation at an Apple Retail Store, and much more.

If you wish, you can share a single Apple ID for iTunes purchases, or create a Family Sharing account.  But I would still recommend individual Apple ID's for everything else.

Family Sharing - Apple Support
https://support.apple.com/en-us/HT201060​

Cheers,