Isn't it all just a business thing? Both have to bring up less input than usual (Mark 70 instead 120, Bob's set is also slightly shorter, and he gets bigger arenas filled than usual?) I am not sure about this but I think financially it is a very good deal for both of them? For example, MK should be less exhausted after these 30 shows than he would be after 30 of his "usual" shows. Costs and expenses (presale, promotion) for selling out 30 arenas in Europe seemed also to be shared. As Bob asked Mark, I still think it is a mercantile thing in the first place and has no artistic reasons - of course the name has to be a big one (like Mark's is in Europe, others are in the US).
As Guy meant, it was never planned to join the stage together, so we should leave this to them... Maybe there will develop something fine. I am sure both artists would not have any plans before the tour and both would just see how it goes, and maybe then anything could happen...
LE