Check the following file. Untrim the hole with the ! _Untrim tool and then try to select one of the two surfaces. You will notice that they appear as a single polysurface instead.
Untrim bug.3dm (114.9 KB)
Yep. I can reproduce this with a simple box with a hole through it if I remove the connecting sides.
Rhino does allow these kinds of structures as valid - you can create them yourself by using NonManifoldMerge - however I guess the command UntrimHoles should automatically check for this and try to fix it…
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This is a quite easy “hack” how to create hollow objects that are usually not allowed by other CAD programs. Great for 3d printing. 
I think this is exactly the usecase this command has been created for.
Yeah, it’s a nice way to trick the program that the object inside is the inner wall of the outer object. Topic’s title changed from “Bug” to “Cool feature”.
NonManifoldMerge was created primarily for - as I recall - making FEA analysis meshes, will not, I think, let you create a hollow object as you might expect because the core object normals will be pointing the wrong way - the best way is to do this with meshes where the normals are allowed to flipped inward.
-Pascal
Please, DON’T remove this feature, because it’s awesome. A few guys I know already found it useful, as well some people on the Facebook group of Rhino, so I guess it has its advantages.
Hey all - the developer reminded me that the way to get the result you want without resorting to the bug and jumping through all the hoops is
NonManifoldMergeCreateRegions- Delete the regions you do not want , like the core in the two sphere exacmple.
-Pascal
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I remember seeing that technique on some tutorials years ago. I find it interesting that there is an alternative way to achieve it through the “hack” I showed, without relying on those two commands.