Not supported yet. see http://wpt.live/css/css-backgrounds/box-shadow-overlapping-001.html
Created attachment 428243 [details] Patch
Committed r277323 (237582@main): <https://commits.webkit.org/237582@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 428243 [details].
<rdar://problem/77842761>