SoSW stat
STR : Increase ALL weapon based Physical Damage. everything which use weapon always use this stat. druid spell which using weapon to attack also use STR. Valiant attack spell use STR too.
INT : All 4 element(fire, ice, wind, earth) spell use this, all wizard attack spell damage, combination of 4 elemental spell damage use INT too. also increase mp
WIS : Light spell damage, healing potency, increase mp too, hero song success rate. i believe WIS also increase resistant to status effect but not sure
VIT : Increase HP. vital for every class. minimum vit 15 is recommended for all classes.
AGI : Dodge rate, hit rate, intiative rate. this stat is important to damage dealer, raise this so that your attack hit.
LUC : instant kill rate, critical rate, chance to inflict status effect, chance to avoid status effect, alchemist spell damage. i believe this stat also affect charm and druid's great wyrm spell

SoSC
STR : Increase ALL weapon based Physical Damage. everything which use weapon always use this stat.
INT : all wizard attack spell damage(black magic tree). also increase mp
PIE: all cleric spell damage(white magic tree), healing potency, increase mp too, sword song success rate, i believe PIE also increase resistant to status effect but not sure
VIT : Increase HP. vital for every class. minimum vit 15 is recommended for all classes.
AGI : Dodge rate, hit rate, intiative rate. this stat is important to damage dealer, raise this so that your attacks hit.
LUC : instant kill rate, critical rate, chance to inflict status effect, chance to avoid status effect, might also affect sword song success rate.

by understanding how stat above works, we can decide which stat to focus on each class.
Valiant : STR
Fighter : STR
Paladin/Knight : VIT/STR
Samurai : AGI/STR
Wizard : INT
Healer/Cleric : WIS/PIE
Ranger : AGI/STR
Ninja : STR/LUC
Alchemist : LUC
Druid : LUC or anything(depends on build)
Dancer : depends on build.
Clocker : depends on build.
Puppteer : LUC i guess
Freeman : whatever

basically you dont want to pick a class with different main stats because that would makes your character stat spread too thin. there is an exception though which depends on how you want to build your character to be. to make it easier to understand, i will give some example below

Bad Combination : Fighter-Wizard is a bad idea because you gonna split your main stats to STR and INT and worse is fighter skill didnt benefit from INT and Wizard spell didnt benefit from STR.
the key is to not combine a class which got different main stat!

Good Combination : Samurai-Ranger is great because both got same stat to focus on. raising STR increases both samurai and ranger skills while raising AGI helps fulfill both katana and bow weapon stat requirement not to mention AGI raises dual wield potency and accuracy too. hybrid Samurai and Ranger also enable you to use Slash 3 from back row
the key is to combine a class which share same main stat!

Unique Combination : Wizard-Cleric/Healer is unique and also great if you got tank to protect them. eventho this combination got different stats but their skills is synergy beautifully. INT and WIS/PIE both increase MP pool, you can cast heal thrice a turn, and you get benefit from mana regen or exchange mana. if you dont want to bring both wizard and Cleric/Healer, this combo is a good option to pick but you gonna need to bring Paladin/Knight as a tanker to protect them.
the key is to decide what kind of role your character want, and looks for possible combination with a skills which synergy well

Stranger of Sword City Revisited
https://www.spoiler.jp/srv/tsurugi/index.php

Savior of Sapphire Wings
https://www.spoiler.jp/game/aotuba/

protip : google translate

i'm not good at creating a guide and also not good with english so sorry if the guide is hard to read.
take this guide with grain of salt. if in doubt, check jp wiki for references.
hope this helps