I agree most DM's wouldn't let you take two half-templates. At the very least you wouldn't get the +8 to strength twice. Hmmm. what about a dragon with a half dragon template....
Well, either way, taking a half dragon template for energy immunity is pretty wastefull, the ECL you are talking about rings of greater protection from energy should do fine, and then spells for immunity after that.
So dropping the half-dragons you have:
Start with a troll (6 HD, +5 LA) ... 5 levels of Wizard ... and 7 levels of Pale master ... and you have 6+5+7=ECL 18 **; CR: 5 (troll) 5 (wizard levels) + 7 (pale master levels)=CR 17 (probably 15 for non synergy of troll and wizard); HD: 6(troll)+5(Wizard)+7(Pale Master)=18
Edit **6+5+5+7=ECL 23
Did Jack Simth have a duplicate 5 in his ECL calculation?
So you are character level 18, but cast as a 9th level wizard. Your attack bonus and damage would be about the same as a 9th level fighter. That makes you a less than ideal wizard, but by selecting utility/protective/enhancement spells you might be a moderate fighter. Immune to most physical damage, but not a lot of hitpoints when someone does start affecting you. With -4 Inteligence/Charisma 5th level spells are probably the highest you will see, so from there you advance as Troll or Fighter?
Your worst enemy would be transmutation and save or die spells, but your fortitude would be your highest save with constitution modifiers. It would be interesting to see, I doubt it would be as powerful as it seems. The big downside is not many of my campaigns would allow for either a troll or a pale master, let alone a combination.