Thanks for your very clear reply Heiner. Yes, looks too complicated for me. I think it would be just as easy to install a NAS running LMS and Plex as using two Pis and two portable HDDs.
Alternatively, I might just stick with my current set up, and sturdy 8 y/o PC that has plenty of internal storage and use that as my server and leave on 27/7. Yes, it might use a bit more power, but its simple and does the job.