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