Well, with the +1 manifester level/+1 spellcaster level each level of Advanced Being, your point becomes more or less pointless. Plus, due to a mechanics problem with 3/3.5E, requiring such a heafty toll on Manifester and Spellcaster classes results in substantially weakened characters. Multiclassing works as written for non spellcasters/manifester. However, when dealing with spellcasters/manifesters, it cripples the characters. You an either take my word for it, or check it yourself. A 20 Wizard/20 Psion will be grossly overpowered by a 40 Wizard, or hell, even a 30 Wizard (or Psion). A 40th level character will be able to be defeated by a 30th level character, due to how spell/power resistances, and DC’s works. This doesn’t work. Now, you cannot completely get rid of one or the other, or else it won’t quite mesh up correctly. So, there’s a comprimise - which was 6th level Psionics and 9th level Magic.
Why did I come up with it like that? Contrary to popular belief, it was not because of Psychic Warriors - that was merely a convinience. It was because, in order to not totally screw the character, it’s necessary that both abilities are not maximum - once again, before you argue with me on this, I strongly recomment playtesting it, or sifting around on boards where people are constantly talking about it. WotC even acknowledged this problem within the book Unearthed Arcana, where they provided a means for caster level/manifester level to keep increasing even when you aren’t raising that class, to help mitigate the effects.
So, one had to be lowered (or else, the whole idea of a character who is the most powerful spellcater/manifester is really thrown out of whack by requiring a max of both at the beginning, simply because of the rules mechanics problems and limitations). Well, in every reference material for 2E I read on Dragons, Avangions, Sorcerer-Kings, etc., they are always referred to as having powerful magic, sure, their psionics are also mentioned, but mostly it is Magic. Even the term “Sorcerer-King” is not “Psionic-King” - it’s “Dragon Magic”, not “Dragon Psionics”, and so forth. I decided to work with the idea that Psionics are used to enhance magic with them, and therefore, are secondary to their Magic. So, keeping with the flavor of the setting, based on the books (both novels and game books) from 2E Dark Sun, I went with the idea that psionics should be the one to take the hit, so as to not cripple the character who is working towards transforming into an Advanced Being (as outlined in the paragraph above).
Now, my question I put forth to myself was, if I was to reduce Psionics, how much would I lower it? Well, I went with 6th level because the Psychic Warrior did not exist in 2e, and I felt that it could be plausable that a Psychic Warrior could attempt the transformation. Besides, for certian Sorcerer-Kings, I rationalized, it makes more sense that they are Psychic Warriors, than Psions (like: Borys, Hamanu), of course, that was a means for me to explain how they got special, next-level swords from Rajaat, and nobody else did - what if the Scourge and the Scorcher were given to Borys and Hamanu (or rather, to Myron originally) to enhance their already more developed melee and martial abilities, while the others had more powerful psionics to their beck and call? This also means I think Irikos and Myron were Psychic Warriors, rather than Psions, for similar reasons.
Plus, I have, just like the Cerebrmancer class, that the Advanced Being Epic Prestige Class advances the character +1 level in their previous spellcasting class, and +1 level in their previous manifesting class. So, within 6 levels, the character is (as a psion) manifesting 9th level powers. Now. Let’s see… as a wizard/psion, this means the character could potentially (ignoring the Cerembrmancer PrC for the moment) begin the process at level 28 (17 Wizard/11 Psion). Now, let’s look at what the official conversion rules booklet would say about converting a 2E character to 3E. You’d take the first class (of the dual-class), ie: highest level class, of course, since both would be the same, we’ll just pick Wizard here. a 20 Wizard would transfer to 3E as a 20 Wizard. The second class, Psionicist, then transfers across at 1/3 of the levels. So the character becomes a 20 Wizard/6 Psion (rounding down), or Character Level 26. in my system, the character would be Character Level 28, two levels higher. I think I got pretty close to the mark on that one.
Sure, the character doesn’t begin as the most powerful psionic character, but there’s other bonuses to deal with here. First, they keep advancing in their psionic class, and become more powerful each level of Advanced Being. Second, I have special rules involving my rewrite of Psionic Enchantments, which are methods and means of combining magic and psionics together to enhance a spell or power as if it is both, plus making it substantially more difficult for people to resist them if they can only resist one or the other. It technically provides more rules for enhancing magic, including rules on how to augment spells using power points, above their normal cap. Plus, being able to metapsionically enhance spells is a handy feature, especially taken into account that it is pretty much on the fly.
The character becomes quite powerful, in actuality, due to several features of being an Advanced Being, much less a Dragon - which also adds in Dragon Magic - that wonderful ability to rip the life energy of animals out in order to enhance their spells even more, the double-whammy, as a player called it, he kills by casting his spell, and then kills more with the spell.
Unfortunately, the beta rules that were released never fixed the problem of a single class character being able to beat an advanced being with multiple classes. The thread is a good read (with a bit of unnecessary acrimony), and you get a good look at how people were thinking at the time.