On this tour, whether people here like it or not, Dylan is the headliner and Mark is the support act. I don't say this is right or wrong and I don't say it as a judgement on their respective abilities, strengths or weaknesses. And I'm not passing an opinion. It is simply a statement of fact. That's how the tour came about and that's how it was set up.
It is to Mark's credit that he agreed to do the tour, knowing that he would be the support act when it is self-evident that he could headline any tour of his own.
Maybe, Mark's appearance in Dylan's set at Bournemouth was a trial to see how it would work out. If so, then it obviously went well enough for both of them to want to continue the arrangement and build on it.
Playing with Dylan is not straightforward. For a start, even though he's only done 14 shows so far and is mostly doing 14 songs a night, he has already included 42 different songs in his sets. Here they are, as compiled by John Baldwin, with the number of times each has been played so far:-
Leopard Skin Pill-Box Hat (14)
Highway 61 Revisited (14)
Thunder On The Mountain (14)
Ballad Of A Thin Man (14)
Like A Rolling Stone (14)
All Along The Watchtower (14)
Things Have Changed (11)
Tangled Up In Blue ( 8 )
Honest With Me (7)
A Hard Rain's A-Gonna Fall (7)
Summer Days (6)
Desolation Row (6)
Blind Willie McTell (5)
Forgetful Heart (5)
High Water (For Charley Patton) (4)
Tryin' To Get To Heaven (4)
The Levee's Gonna Break (4)
Don't Think Twice, It's All Right (4)
Spirit On The Water (4)
Not Dark Yet (3)
It's All Over Now, Baby Blue (3)
Love Sick (3)
It Ain't Me, Babe (3)
Just Like Tom Thumb's Blues (2)
Simple Twist Of Fate (2)
Watching The River Flow (2)
Beyond Here Lies Nothin' (2)
Can't Wait (2)
Make You Feel My Love (2)
Mississippi (2)
The Lonesome Death Of Hattie Carroll
Cold Irons Bound
Visions Of Johanna
Nettie Moore
Shooting Star
Man In The Long Black Coat
This Wheel's On Fire
I'll Be Your Baby Tonight
Jolene
Forever Young
Workingman's Blues #2
John BrownNote that "John Brown", as performed at Mannheim with Mark on guitar, was making its first appearance on the tour. It's an old Dylan song, not on any of his old Columbia Records albums and never performed live for decades but then resurrected in recent years - and not one of his best songs, in my view, but that's another issue. For Mark, it was unlikely to have been a song with which he was particularly familiar, though some might suggest otherwise.
So that's the first difficulty, the number of songs and unfamilliarity with some of them. The second is that, for Dylan, any rehearsed arrangement is rarely fixed. He doesn't always stick to it: keys change, words change, verses are swapped around, harmonica pops up in different places and so on. This is why his band watch him so carefully because it's their job to keep up with him, whatever he does and whereever he goes. I can even recall a show where Dylan has wandered off-stage for a while in the middle of the song and the band just had to play on until he reappeared. It's not disrespect on Dylan's part, it's living with the moment. Sometimes it works wonderfully and, other times, it does not. For Mark, it cannot have been straightforward.
I guess that the guitarist in Dylan's band to whom you refer is Charlie Sexton:
http://en.wikipedia.org/wiki/Charlie_Sexton