mystral: Not really. Arcane Failure isn't implemented in BG 1 or 2 to begin with (equipping any armor just disables arcane spellcasting) but more importantly, in most fights you either don't need armor (against spellcasters) or you don't need to cast spells in the middle of a fight.

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.