A tool to preview two ui files at the same time would be really good because when developing in several branches there sometimes comes conflicts with ui files. At the moment, the best tool to merge ui files is a xml merge tool. If the ui file is big and compilicated then it might be difficult to understand it (unless you are the one who did those changes).
It is really difficult to imagine a graphical merge tool with a good user interface but a tool to preview two ui files shouldn't be a difficult task. In Qt Designer there is a functionality to preview the current project, already.