Read Life and Times of a Bhaalspawn to gain an appreciation for what's going on here. Or don't. Suffice it to say, Billy took Viconia's epilogue hard.
Narrator: Hey there, Billy! Why the glum face?
Billy: I am death given form. All I touch turns to ash in my grasp. Those I love, perish. My love, Viconia, is gone, and I am torn between throwing myself into dark feelings of revenge, and casting that vengeance aside to give my son a better, more peaceful life
Narrator: Wow, Billy! That sounds super interesting! But you know what else is interesting? Dual Classing!
Billy: Leave me to grieve for my departed wife.
Narrator: First of all, let's talk about all the Dual Classing options. You have Fighter > Mage, Cleric > Mage and Thief > Mage!
Billy:... What about the other options?
Narrator: Don't be silly, Billy! Nobody dual classes to anything but a Mage!
Billy: What about dual classing from a mage to something else?
Narrator: AHAHAHAHAHAHA Christ, Billy, are you serious?
Billy: Cleric's are good. Viconia was a Cleric. I loved how she would smile slightly when healing someone, if only out of satisfaction gained from her own talents as a Cleric.
Narrator: Let's be serious, Billy! First off, let's talk about Fighter > Mages! Like you, Billy!
Narrator: I need you to show some enthusiasm, Billy.
Narrator: That's the spirit! So, Fighter > Mages are used for several reasons! First, to gain higher HP! Second, to actually be able to use weapons as a viable alternative to magic! Thirdly, to gain access to Fighter gear as a Mage!
Narrator: There are several kits to be used as a Fighter > Mage. Which do you think is best, Billy?
Billy: The one which would give me the power to save my wife.
Narrator: You have to pick one, Silly Billy! But the real answer is: Berserker! Did that surprise you? I bet you thought it was going to be Kensei! I know how you love being a Kensei > Mage!
Billy: No. Once I saw my wife waste away in front of my eyes, I realized my own powerlessness. All my training as a Kensei. All the lessons of magic that Gorian imparted to me in my youth, that I only began to capitalize and use after Irenicus had tortured me, mastering the arcane as a Mage... None of it could save Viconia. I know well my own flaws.
Narrator: I love how you wove your backstory and reasons for dual classing in there, Billy! But yes, Berserker > Mages are the best Fighter > Mage dual class, due to the immunities inherent in the Berserker's Rage ability. The negligible bonuses from Kensei simply aren't as valuable as that, and Wizard Slayer > Mage is just plain horrifying to even imagine. A plain Fighter > Mage dual class is certainly solid, but lacks the advantages of Kensei/Berserkers.
Narrator: Now, Cleric > Mages are a whole different kettle of- Billy, why are you trying to sneak away? You aren't a Thief > Mage, Billy! And why would you want to be, considering the game is overloaded with them anyway! I mean, wow, there's two of them and a Thief/Illusionist multiclass in-game. But I can see you're tired, so maybe we should talk about a bad dual class to cheer you up!
Billy: I need to sleep on whether or not to launch a vengeful crusade against the drow.
Narrator: Oh, you absolutely do that.
Narrator: What, didn't you finish reading the epilogue?
Billy: No, my tears blurred the screen.
Narrator: Didn't I tell you before? Viconia's epilogue is pretty harsh.
Billy: Hypothetically, what would have happened if I'd gone with Aerie or Jaheira instead?
Narrator: You'd have lived happily ever after, heck, Aerie would have even met her parents again!
Billy: Then... why only Viconia...
Narrator: I don't know, Billy. But you know what I do know?
Billy: What? What do you know? Is it about Viconia? Is she at peace in the afterlife?
Narrator: I know all about Dual Classing! So let's talk about Thief > Fighters now! Yaaay!