Is it completely broken now (in version 530)? For me, it doesn’t work at all. The command simply does nothing.
I have fixed it. Thanks
Georg, can you provide a new build with this fix soon? The Command that Tim mentioned does actually work but it really screws things up.
For your reference, here is what I have run into:
OS X 10.8.5, Glyphs 1.3.26 
SHORT DESCRIPTION: Using Cmd-K to send a selection to the Background is causing the Foreground outline to take on some of the properties of a component. If I edit the Foreground outline, the Background outline updates immediately to reflect the edit.
If I send a selection to the Background (Cmd-K), then move the selection in the Foregound, nothing shows to be in the Background layer while in the Foreground layer even though a Cmd-B shows it is there and selectable. A test shows that if I move the selection in the Foreground layer, the background copy moves with it even though it had been deselected. Any edit to the Foreground outline causes the Background outline to update immediately, reflecting the edit.
If I send a selection to the Background (Cmd-K), then go the Background layer and select it and delete it, all nodes on the selection in the Foreground layer disappear, leaving it not selectable when I double-click on the outline.
As regards #2 above, if I click on the anchor in the Foreground and delete it, it also deletes the Foreground outline (which has no selectable nodes). If there is no anchor to begin with, then the nodeless outline is not selectable and cannot be deleted.
Instead of using Cmd-K to send the selection to the Background, if I do a simple copy and paste of the selection to the Background, none of the above happens.
[EDIT] Apologies for the duplicate report; I just noticed the one from another user earlier.
I uploaded an update that should fix this.