The 2nd NAS is set up to not show its content in LMCE. Also I wouldn't go near a RAID setup, I have never liked the idea of data being automatically mirrored, also every time I read about RAID it's usually in the context of somebody having issues with it. I use rsync like possy says to backup, run by a cronjob. This way you can decided which folders are backed up incrementally, which ones are mirrored, etc.
Like you I didn't want to back up all of my movies. However a nice compromise is to have the cronjob also run "ls" on the directory in question and put the output into a text file with a date stamp. That way if the hard drive goes down, I at least have a list of what I had.