Small edit done by me that nerfs enemy talent level scaling across the board. I take no credit for this great rework of the original Multi-Class challenge, all credit goes
    to AscendingPhoenix, find the orignal mod here: https://te4.org/games/addons/tome/dual-class-challenge

Allows the player to pick two classes, but the difficulty scales upward. The mod dramatically ramps up the difficulty level to compensate. (Original by StarKeep, forked mod by Raenarill).
You advance twice as fast, and gain six talent category points: at 10, 20, 30, 40, 50, and 60. You gain normal stat points until level 51.
BLITZ MODE: You gain two thirds more talent points. After level 66, you gain one class talent point per level and one generic every two out of three levels.
CLASSIC MODE: features base game talent points until level 100.

CLASS EVOLUTIONS: You can swap your first and second classes in the Game Options tab for evolutionary purposes.
WANDERERS AND ADVENTURERS: These are only supported as the FIRST class.
QUANTUM SLIDERS: Now supported as the second class.
EXPERIENCE ADJUSTMENT: You can now adjust how much experience you are gaining, from 50% all the way up to 500%.

TIP: Good equipment takes the usual time to show up, so you probably will see it at about lvl 50 with dual classing enabled.

In order to activate this addon, look at your Game Options for a new tab. Enjoy.

Allows the player to pick two classes, but the difficulty scales upward. (Original by StarKeep, modded by Raenarill).
You advance twice as fast, and gain two thirds more talent points. You also gain six talent category points: at 10, 20, 30, 40, 50, and 60.
After level 66, you gain one class talent point per level and one generic every two out of three levels.

In order to activate this addon, look at your Game Options for a new tab.

Allows the player to pick multiple classes, turning all enemies into Randbosses as a counter-measure.

In order to activate this addon, look at your Game Options for a new tab.
EXP penalty you suffer from using this addon has been modified, from 0.5 to 0.85 (You only lose 15% EXP now)
Original Author is StarKeep, all credits go to him, I do not take credit for any of his work, I only modified the EXP penalty. - Rae
Updated Version 1.4.1:
- Possessor and Adventurer can now be chosen in the Multiclass menu