Split-Class Penalties

Back a few posts ago, I suggested a simplified approach to multiclassing which might be better classified as split classing: if you add a second class, your prime ability score, for purposes of experience point adjustments, is your lowest score of the prime abilities for your two classes, divided by two. Thus, if you have two 18s, you could pick up a second class and advance at a normal rate (no modifier,) because your effective prime ability score is 9. But if you have one score that is 17 or less, you will advance slower, because you will have a -10% or -20% earned experience modifier.

In the comments on the post, Scott Anderson asked:

So adding a second class is like gestalting but with a steep XP penalty? That seems workable. The lower the level, though, the more disproportionately powerful the class is- unless the XP penalty is nearly prohibitive.

I can't really answer the question  about gestalting, since  all I know about 3e's gestalts is that there was something called "gestalts". I'm not sure where the comment about disproportionate power at lower levels comes from, though.  Is a 2nd level Fighter/Thief disproportionately powerful compared to (a) a 10th level Fighter/Thief, (b) a pure 2nd level Fighter or Thief, or (c) something else I'm missing? Since the Fighter/Thief uses the Fighter XP table (with a probably XP penalty,) but has Thief hit dice (effectively half the hit dice of a Fighter of the same level,) I'm not seeing a gross power imbalance. What am I missing?

What does concern me is how to handle Fighter/Magician. I was working purely in terms of Fighter or Magician + Talent, where "worst XP + worst HD" makes sense as a penalty, but the M-U is already using the worst XP table and the worst HD table, so a F/M splitclass is barely penalized at all. The options to fix this are:

1. Don't allow F/M, only allow F/T and M/T;
2. Allow F/M, but double or otherwise modify the XP needed;
3. Impose another limit on F/M, such as reducing ability in one or both classes.

The third option is kind of hard to do, if you are allowing a split-classing option after character creation (Fighter decides, at 2nd or 3rd level, to pick up some magical ability.) That makes banning F/M split-class characters, except for elf-style multiclassing, an attractive option.

If, however, you limit F/M split-classing to piecemeal additions, it might be doable. Under this scheme, a Fighter can learn spells and a Magician can learn weapons, one at a time. Doing so makes you an F/M or M/F split-class.

• Fighter/Magician characters are standard fighters, but can cast half as many spells as a normal Magician. They can never learn to create magic items, other than scrolls or possibly other consumables.
• Magician/Fighter characters are standard magic-users, but can learn to use weapons and armor and fight as a Fighter of the same level. They can never learn to fight multiple opponents simultaneously, nor can they establish a barony and collect taxes.
• Both split classes use the M-U XP and hit dice tables. The prime ability score is the lowest of Strength or Intelligence, halved.

(This makes the Fighter/Magician roughly comparable to the cleric and perhaps should replace that class altogether.)