We like to use B2B long runs for runners and run/hikes/climbs for mountaineers in the the specific period as a way to mimic most of the demands of the event without impacting normal life too severely. I do not think there is a magic balance of distance on the two days. The important thing is to begin day 2 carrying some fatigue. This will make the second day’s training effect especially powerful and more similar to what you’ll feel during the later stages of the event. I normally ask my athletes to shoot for close to 50/50 distribution on these days. But I do not have an science behind that.