I'm afraid this is not possible on a non-jail broken iPad. Having said that, though, depending upon what activities you want to keep separate, it is possible to have, say, multiple email accounts and, with some third-party apps, separate diaries. But in the Windows sense of multiple log-ins, it's not possible. One iPad per person was Apple's vision....
While you can't have separate account control ... you may be able to have separate email accounts. I have both a Yahoo and Gmail accounts. Let's say she has Yahoo and you have Gmail. You can easily to to those sites and let the e-mails reside on their respective servers. It is a work around idea ...
As for documents and writing, perhaps Google Documents might work.