I thnk there is something inherently wrong with someone being able to say that they are gaining caster level strength equal to a character who is actually taking a level in a said class. There has to be a consequence for taking levels in a different class, and I don't think that reduced spells known and reduced spell slots is adequate.
Amen. If you allow CL = character level, you'll have people taking umpteen levels in a fighter class and 1-2 levels in a spellcaster class, then being able to toss off 5 magic missiles at CL 1, or have your spells last forever. You're a neophyte spellcaster - you shouldn't be able to do things like that. Sure, the other levels should add
some benefit in terms of power, but not that much.
Now, I also agree that the multiclassing system penalizes multiclassed characters too much. But I like the above level I suggested. A rogue 3 / sorcerer 12 would cast their spells at a caster level of 14, not 15. A rogue 6 / sorcer 9 would cast their spells at acaster level of 13, not 15.
Why not apply the fractional system to spellcaster levels too? I took the UA system and split it up - 1:1 (full spellcasters), 1:3/4 (bards and adepts), 1:1/2 (half-casters) and 1:1/4 (non-casters). Then you just tally up the fractions from each class and apply them however you want (I haven't found a better system than the one I came up with, so I'm using that one).
The question is are the level 11-20 abilities of either class combined with its 1-10 abilities significantly more powerful than combining those 1-10 abilities with the 1-10 abilities of the other class.
Considering most classes (including PrCs) are badly front-loaded, I'm not sure that you could accurately make that comparison.
A player who split-classes must decide which class is his primary class and which is his secondary class at the start of play.
This sentence is a deal-breaker. I try not to plan out my character's advancement from L1 - in fact, my characters usually go in unexpected directions during play. My favorite PC started as an adept, gained wizard levels, then became a necromancer who specialized in spirits (I had to make a custom PrC for him). You can't plan for things like that.
What you're suggesting, though, is very similar to the gestalt rules.