I don't think it is a limitation of iOS, but not offered by the system as a standard yet - I think the developer would have to make their own unique solution and the issue might be that that is a lot of work and, possibly too, Apple might then offer it as a new feature in iOS 11 or some iPad focussed point upgrade.
I did see this recently as a feature of a word processor app, but currently can't find the link I saw . I can't see it listed here: http://brettterpstra.com/ios-text-editors/ but then again, neither is Omni Outliner.
Personally, I would like two Omni Outliner docs open side by side that I could edit simultaneously but I realise now, I might be in a minority of users. It's strange how our own preoccupations seem obvious, common and popular but surprisingly, when shared more widely, we find that others are completely indifferent to them. This may be the real reason why (nearly) no one offers this as a feature., it's too niche.