Sadly I've never seen them play together.. I missed the whole thing, but I do enjoy their styles.. separately.
I've seen Sasha play live 4 times, 3 of which were in San Francisco, and only one was memorable, and I've see John like 9 times .. all of which were pretty amazing.
Now.. after seeing them both at a festival couple of weeks ago, I have to say that the "mini journey" you are talking about is very much what I though when I saw Sasha's set. He managed to make that outdoor venue feel like a small intimate club.. and it was amazing.
I managed to record his whole set, but since I can't post links.. just search for
Sasha live at EDC Las Vegas 2013 (full set)
John's set was great as well.. hardcore, straight to the point.. which is what you would expect from a short festival oriented set ..
But then I went to his Bedrock after party (the same night) and that was.. well.. way better than anything I saw at the festival, not even close.. but it took about 4 hours, where at the festival he only had 90 mins or so.
Search for: EDC Las Vegas 2013 - John Digweed (full set)
Which brings me to the question.. why do they need to play b2b ? Can't they just do.. Sasha opening for about 2 hours, and then John comes in and smashes for another 3-4hrs ?
After seeing them both in the same setting, I think that would work beautifully