Students can create an online journal using Evernote and then they can edit each other's writing pieces in their journals.
Evernote is an excellent app that can be used on the iPads in your classroom. Using Evernote the students can easily peer edit one another's papers. This will be beneficial to the slow writers who can type faster as well as the students who need to frequently look up the spelling of words. They can have one tab open to Webster dictionary and be editing a peer's paper via Evernote!
Teachers can use Evernote too! We are always working as a "team" with our grade level teachers and this way we can all contribute to projects and assignments yet produce just one copy. It is very beneficial for working collaboratively. It's also easy to use. I've created notebooks with coworkers and we can all easily access the notebooks and make changes to it as necessary.