Honestly, I really think we witness a very subtle development here. First both artists start the tour and aim to optimize the interval time for the roadies and let Mark get warm with the band and tour. Then next step, Bob asks Mark. Guy standard answer always is "let's see what happens/nothing's planned yet". So we might be able to see more real "duets", maybe some longer part in the middle of the show with one, two, three songs etc. Mark definitely is
too modest to ask Bob to give him more time, but maybe it works so well and Bob has some fun with it so there will be more in the next few weeks.
LE