I'm quite sure, that it won't work. I guess, that an application has to support multimonitoring (it has to recognize the second monitor). Otherweise you can only stretch the application to fit two or more screens. I'd surpised, if LOTRO would offer this feature.
But one solution might work in the near future. It might be possible to stretch the resolution to fit both screens. And soon it should be possible to place UI-elements everywhere you like. But theres still a big problem: How can the viewport be centered on one screen?