wait up....
not a very good idea to place them on the exact same spot/size, would be better if you put the illusionary on the inside and the skybox one on the outside (is how i do it), that way you prevent any problems with it
Why? In my map there are no noticeable problems yet. The skybox doesn't cast light through the illusionary even though they share the same space. Honestly I'm really new so I would love it if someone could tell me why you're not supposed to do this. I talked to drexen and he didn't come up with anything so it should be fine. ^.^
Also func_illusionary is pretty much just a more specific version of func_brush, says so in the wiki entity description here
https://developer.valvesoftware.com/wiki/Func_brush