Probably the easiest would be to add a cross-platform (separate Windows and Unix implementations) named pipe class, that could be read and written to like a file. Adding a new library could be overkill.
However note that in case of a "sub-editor" you would be loading the resources multiple times, resulting in heavy memory use, so in an ideal case I wouldn't recommend that approach.
Moving a graphics context between windows might not be impossible, however Urho's frame loop is not well-fitting to multiple window rendering, since it does first a separate render update, then a render later. Unifying the render-update and render operations would be preferable for this (also to prevent situations like scene destruction between update & render resulting to a crash), however it could lead to performance loss or subtly break applications which rely on the current behavior.
Also, if we had actual multiple windows support, then we'd also have to consider multiple window input, which could cause larger scale API changes, so the IPC approach (though wasteful) is still the easiest to get running.