I use multiple outlines as part of my workflow. It would be terrific if I could open more than one at the same. It would be even better if the open outlines were listed in the side bar or somewhere visible and convenient. I know I can get the same result in Omnifocus which I have been using several years. I really like Outliner 4 and would prefer switching to it.
Did you intend to post this in the OmniOutliner for iPad forum? Because OmniOutliner for Mac, like virtually all document-based Mac programs, lets you have multiple files open at the same time.