iPhone Back up

Not that I'm aware of. Unless someone has a better idea, I think your best bet might be to have a shared Hotmail (or similar) account which contains all your Contacts. That way, you can add the Hotmail account under the Mail section under Settings and just choose to Sync contacts to both phones.

My other half and I have a similar arrangement with a Google calendar to allow us to share calendars easily.

One (or both) of you turn off the contact synch. I use googlemail through the exchange service and that does my contact back up. The up side of that is if you move away from apple, you can still get your contacts back.

Not sure if this will help or not. 

But did you know you  can share a single Apple ID (so that you can have the same apps etc) but you can have separate icloud IDs for backing up (to the cloud). 

See 

http://www.macworld.com/article/163006/2011/10/how_to_configure_icloud_if_you_share_an_apple_id.html

Miles

Had the same problem and set up new iCloud accounts for 

me and the wife but left the main apple Id on both our phones for iTunes purchases.