Often there is a problem with Crown Moulding and the Baseboard not flowing around the end of free standing walls, such as when there is just an opening from one room into another. Previously I have made a thin wall and drawn the three sides of the wall as if it was thin room. I found however that you can actaully use a regular wall, draw the wall as if a room and then drag one wall into the other. The two walls will actually merge but still have an 'end' that retrain the mouldings. It remains two walls, so will not accept doors and windows but if they are not needed, then it easily solves the moulding problems.
It is probably best not to drag it completely through to the other side, So that the two walls are exactly in the same space, so that the two textures (if different) do not get mixed up. Use the Nudge set to a small measure, to make these fine adjustments.
Allan