Before unlocking a skill, each possible class combination has to do a quest to become "worthy" of basically becoming partial to another class.
So, a Lord Knight want to share with a High Wizard would need to quest.
If they restored a backed up character, they would need to do the quest again for that class if they wanted to share with a High Wizard.
The quests are the same for each class, and they have to be done for every class you want to share with. The problem is coming up with the quests.
That's just an example, hopefully it's clear enough.
Here's the combination list:
//------ 2-1 and 2-2 Advanced
Lord Knight & High Wizard
& High Priest
& Assassin Cross
& Whitesmith
& Sniper
& Paladin
& Professor
& Champion
& Stalker
& Creator
& Clown/Gypsy
High Wizard & High Priest
& Assassin Cross
& Whitesmith
& Sniper
& Paladin
& Professor
& Champion
& Stalker
& Creator
& Clown/Gypsy
High Priest & Assassin Cross
& Whitesmith
& Sniper
& Paladin
& Professor
& Champion
& Stalker
& Creator
& Clown/Gypsy
Assassin Cross & Whitesmith
& Sniper
& Paladin
& Professor
& Champion
& Stalker
& Creator
& Clown/Gypsy
Whitesmith & Sniper
& Paladin
& Professor
& Champion
& Stalker
& Creator
& Clown/Gypsy
Sniper & Paladin
& Professor
& Champion
& Stalker
& Creator
& Clown/Gypsy
Paladin & Professor
& Champion
& Stalker
& Creator
& Clown/Gypsy
Professor & Champion
& Stalker
& Creator
& Clown/Gypsy
Champion & Stalker
& Creator
& Clown/Gypsy
Stalker & Creator
& Clown/Gypsy
Creator & Clown/Gypsy
//------ 2-1 and 2-2
Knight & Wizard
& Priest
& Assassin
& Blacksmith
& Hunter
& Crusader
& Sage
& Monk
& Rogue
& Alchemist
& Bard/Dancer
Wizard & Priest
& Assassin
& Blacksmith
& Hunter
& Crusader
& Sage
& Monk
& Rogue
& Alchemist
& Bard/Dancer
Priest & Assassin
& Blacksmith
& Hunter
& Crusader
& Sage
& Monk
& Rogue
& Alchemist
& Bard/Dancer
Assassin & Blacksmith
& Hunter
& Crusader
& Sage
& Monk
& Rogue
& Alchemist
& Bard/Dancer
Whitesmith & Hunter
& Crusader
& Sage
& Monk
& Rogue
& Alchemist
& Bard/Dancer
Hunter & Crusader
& Sage
& Monk
& Rogue
& Alchemist
& Bard/Dancer
Paladin & Sage
& Monk
& Rogue
& Alchemist
& Bard/Dancer
Professor & Monk
& Rogue
& Alchemist
& Bard/Dancer
Champion & Rogue
& Alchemist
& Bard/Dancer
Stalker & Alchemist
& Bard/Dancer
Creator & Bard/Dancer