I know that this is rather a weird rec request, but i do want to know about songs you guys think go nicely together.
I've been listening to Mitochoncrion (Death/Black) and Low at the same time (Slowcore, Indie Rock pretty much) and I like the result.
Some songs that lack ambient sounds are nicely supplemented by Low's slow and serene sounding.
The hard part is finding songs in the same tempo.
Anyone tried that already?

Rosetta released a two-disc album that's meant to be played simultaneously, I think it was Galilean Satellites. This website might be of interest too, it continually plays the sound of rain falling, and sounds pretty sweet with almost anything.