Assuming you are wanting to keep most everything else separate except for the calendar then you would go about it one of two ways.
-Create a 3rd iCloud account (assuming you have one each already).
-In settings, under mail, add a new account. Select iCloud.
-Enter in the information for your 3rd account.
-Turn on calendar.
-Repeat the last 3 steps on the other device.
Alternatively you can do this same thing with a good account and shared /subscribed calendars and using Google mobile sync through exchange.
The reason I say a 3rd iCloud account is because the first iCloud account you add to your device becomes the main and only it can have a backup / use photo stream / and document storage. If you want to keep these things separate you each need your own (so you each get your own 5gb for backup) then you need the third for shared assets.
Another way is to each have your own, and then add each others as a second account. For this scenario you have your own account, she has her own, then you add yours to her device as a second iCloud account and just turn on calendars.
Ok ignore all that. didn't realize you could share icloud calendars that easily from icloud.com. Today, I learned something.