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I have several NBMs here that came in sets, e.g. a library NBM and then the actual netbeans implementation, and I had to install both for it to work. If I was the author and wanted to upload the two together to the portal, I currently couldn't. The update center deals with that case already, when you select one thing it automatically adds all dependencies and installs them along with it.
This is now supported by uploading a zip file.