stefangs:
The problem of "feathers all sides" does not arise if the half-frame masks are made correctly. You only need to make one. It is most simply made from slug. Crop 50% from left. Apply video filter "Negative". Export one frame of that slug as a tiff. But you can also make a half-white, half-black tiff in Photoshop.
When putting your split screen together apply the Image Mask filter to one shot. Drag the tiff into the filter's little window. Then apply the Mask Feather filter, setting the softness to the desired level. Do the same to the second shot, except with "invert" selected in the Image Mask filter. Put your masked shots one above the other in the timeline and choose Modify>Composite Mode>Add.
FCP7 knows not to feather masks at the frame's left, right, top, or bottom edges.
Dennis Couzin
Berlin, Germany