I don't know what you mean by "join the two sets", but moving photos that have been imported into Lightroom is easy.
Move the photos using your operating system, keeping the folder structure unchanged, to the external drive. Then in Lightroom reconnect via Adobe Lightroom - Find moved or missing files and folders
Sorry I may not have been clear enough - what I meant is that I would like to access my Aperture images via LR5, so have just the one LR5 catalog for all my images.
If my Aperture originals/masters are stored on an external HDD, can I simply just "import" them into LR - the existing catalog - and then move the whole thing off to another external HDD?
what I meant is that I would like to access my Aperture images via LR5, so have just the one LR5 catalog for all my images.
This is a goal that I would recommend!
However, I can't really state if that's what you need to do with your photos in Aperture, as I have never used Aperture. I'm sure someone else who does use Aperture will be along soon to comment, or you can try your favorite search engine.