As shown in the following image I apply simple box-shadow CCS to both left and right-embedded views inside a Split Container but the left one doesn't accept box-shadow.
Both embedded views use the same view.
Also if I change the split to vertical mode the top view shows this behavior.
Have you tried the style property overflow: visible ?
Of course. Even without that because we apply the CSS on top of the splitter itself this doesn't matter but for insurance I did it.