No Time For Maiden Astraea - Interstellar Demon's Souls 

Interesting idea! IIRC, the remade version isn't quite as regular as the original, and uses strings instead of harpsichord, so I don't know if they would work as well together. Mashups aren't all that intensive to make though, it's basically just crossfading, pitch shifting, and mixing two tracks together, so if you (or anyone) hears them together and likes what they hear, go for it! On a different note, I've thought about making a couple of my own reimaginings of the Demons Souls OST, because people didn't seem too happy with the new ones, which I think encapsulates the really interesting artistic conundrum of remaking a soundtrack.