Sorry for taking so long to get back to you. I've been knee-deep in code for another project.
If you want to specify multiple reviewers in the dpn_reviewers variable, you will need to serialize an array of addresses and put the serialized information into the database.
In the current version, there is no way to specify multiple email addresses from the text input on the post/page edit screen. However, I uploaded a new development version this morning (you can download it from http://wordpress.org/extend/plugins/post-revision-workflow/download/ under the "Other Versions" heading) that should allow you to specify multiple email addresses separated by semi-colons in that input.