First I'm assuming that by saying that you are using the OneUI that you really mean that you are using the ExtLib Application Layout control.
I got rid of the OneUI and Application Layout some time ago and built my own structure. It is actually very simply a set of facets that I put my content into. In addition now I do everything using BootStrap. Having the same backend document open in multiple instances of a session would appear to me to be pretty dangerous. I have used various tabbed table structures to display different parts of the same document and have essentially come up with a simple solution. I create a panel for each section of the document, and a navigator that sets a viewScope.selectedTab = "Header" where the panel has a style viewScope.selectedTab == "Header" ? "" : "display:none" I don't use the visible property because it does not just make the panel visible but also unavailable so the save does not save the content from different panels, which I'm guessing is what you want to do. Takes a bit of time to create the navigation and panel structure but once done is easily modified.