Of course, you could do a Caster(Mage/Cleric) and a Combat Expert(Fighter/Thief)
The concept behind a fighter/wizard multiclass isn't just to have an arcane spellcaster with higher hp and ac, it's that lots of arcane spells are self-only buffs that greatly improve your survivability (mirror image, stoneskin, protection from magic weapon, spell turning, etc..).
In other words, in many fights you should mostly play your fighter/wizard as a fighter that self-buffs before big fights to become pretty much invincible. A fighter/wizard is a far better tank in BG than a pure fighter, or any other class combination.
Later on, when wizard spells become better offensively, you can just sacrifice some AC (just buy the AC 3 bracers or something) for spellcasting, AC is not important if you kill everyone quickly and/or you can just make yourself immune to physical attacks.