Seriously.. hoard lenses not camera bodies.. This is especially true with digitals. I'd sell it to free up funding for lenses or another NEX if the system does meet your needs.
If you are intend on using it as a backup, you should go on a quick trip with both and see if it works for you. My guess from personal experience that it will not work out. Two different user interfaces (to remember and do a mental context switch), two sets of batteries, two sets of chargers, two different post process workflows, even two crop sizes... etc..
I loved the Epson R-D1.
I also love shooting with the M8.
The R-D1 is about as close to a modern classic digital as you are going to get; You shoot as if it were the same film Voigtlander Bessa Rangefinders from which it was built on. LCD hides, analog dials, even still has a manual "film" advance lever. I absolutely loved it. I forgot that its a digital at times. But in the end, it was a royal PITA to use as a backup to the M8 for all the reasons mentioned. I ended up selling it to fund an M9.
Now if you keep it for nostalgic reasons... or just to have another camera... etc.. then sure keep it. Its why I still have my G1 even though it sees little use, return is small, and I purchased it dirt cheap (wife uses it more now). It introduced me to m4/3rds format so it served its purpose. I figure that this isn't the case since this thread exists and a question is being asked. Trust me... I think at one time, I had 8 digital bodies (don't ask... lol).
IMO, there is little in common between the E-P1 and the original Pen from which to call it a modern classic. The whole user experience is different. Never the less, it doesn't take away from the fact that the E-P1 is still a good camera.. just don't "hoard" it because it "might" be a classic someday.