This isn't going to help for your existing Calibre ebooks, but in the future the optimal method might be to download using K4PC and then import into Calibre using the DRM-stripping plugin. You can likely do this for all your existing purchases (there is typically a 6 device at a time limit per ebook). The resulting DRM-free MOBI will work on both your Kindles, and you can update the metadata once.
Apprentice Alf's Blog is a good place to read about DRM stripping techniques.
The legality of stripping DRM in some countries is unclear. However, as a practical matter no one has ever suffered legal consequences due to stripping DRM for personal use only (e.g. to edit metadata once, rather than once per device).