The browser window method doesn't seem to work when copying content between courses. However it seems to be possible (to some degree at least) by export-import if you're prepared to fiddle a bit.
I was recently able to update the relevant json files (contentObjects [pages], articles, blocks, components) in an exported course with the desired new json content, including new pages, then import successfully with all the new content intact and previewing correctly.
I haven't got round to testing with different assets, but I'd imagine you'd similarly update assets.json with the new info and paste any new asset files into the assets folder. Plugins should be common in both courses before exporting.
I was using AT 0.10.3, framework 5.6.1.