Currently this is possible with the Android app, but it is still a work in progress and there are some limitations. One of the connections must be your primary server and will have full capabilities; the secondary server won’t have all the features of the app available.
Are there any plans for the future or is there maybe a way to approach my goal to use two openHAB Systems on one iPhone, which I am not thinking of right now?
Don’t worry so much about apps. The new MainUI renders very nicely in mobile webpages and you can simply have shortcuts to each of the different OH instances.
You could use the Remote OH binding which allows you to connect one OH instance to another. You would have to create items in the primary OH for all the things you want to control in the second OH and build them into pages or sitemaps on the primary instance for simple access.