ICloud :: Two IPhones To Share Calendars But Keep Mail / Contacts Separate

I am getting a new iphone for my wife and am unsure whether to create a second icloud account or stay with one. I know that we can use the same iTunes ID for apps and music, so no issues there. But I also want our phone calendars to sync with our Mac (already connected to my phone via icloud). My preference is to have two icloud accounts with linked calendars so we can each have 5gb of free storage. Is that doable with shared calendars on the phones and the Mac?

If we must be on the same account, how to separate out our mail and contacts? We are not using the @me account for mail, so my guess is if we use a separate email account in respective phones, our mail won't get mixed up. Is that correct? We already have multiple groups in our address books. Can we select multiple groups to sync one phone and others to the others? Also, can we access shared notes?

Info:
iPhone 4S, iOS 5.1