When playing osu!mania you can set certain samples to play in background (the ones playing the music), however doing so will cause the music to mix together with different BPM on Double, or if speeding up using Replay, making it sound very strange. I don't think this would be a big deal, converting a double speed version on first import of the beatmap (maybe setting a flag?), or maybe on some PC's rendering the samples in real time.