[Faceman]

If a valid multiclass like fighter/mage exists that means that the dual class is also possible with every possible kit of the first class (with special exceptions like the obviously forbidden Wizard Slayer/Mage). So there is the possible Berserker/Mage as well as the legal Necromancer/Fighter (Necromancer must be the first class).
Specialist Mages can dual-class just like normal Mages can but Sorcerers cannot because they are not a mage kit but a completely different class.
Sorcerers in BG2 are completely different from specialist mages.
They cannot copy spells to their mage book from scrolls but choose spells with each level up (so their spell selection is rather limited). However they do not need to memorize spells but have all of their spells available after resting with only a maximum for each level.
Example:
Your sorcerer knows the following lvl 1 spells: Magic Missile, Grease, Armor, Chromatic Orb and has 4 spell slots on lvl1
After resting he is able to cast:
4 Magic Missiles
OR
4 Chromatic Orbs
OR
4 Grease
OR
4 Armors
OR
2 Magic Missiles and 2 Grease
OR
1 Magic Missile, 1 Armor, 1 Grease and 1 Chromatic Orb
OR
1 Grease and 3 Chromatic Orbs
OR
1 Armor and 2 Grease and 1 Chromatic Orb
...

So he can cast 4 lvl 1 spells and choose from every spell he knows (which he picks on lvl up).