If my wizard applies Energy Admixture (Complete
Arcane, page 78) and Maximize Spell to the same spell, does
he get a spell that deals double its maximized damage
(maximum normal damage in two different energy types)?
If he also applied the Twin Spell feat (Complete Arcane,
page 84) to the same spell would it duplicate the entire
effect?
Yes and yes. Assuming your character can cast 13th-level
spells—the slot required by a 2nd-level spell affected by these
metamagic feats—a scorching ray so affected would create two
sets of three rays each, with each ray dealing 24 points of fire
damage (4d6, maximized) and 24 points of a second energy
type (as appropriate for your Energy Substitution feat), for a
grand total of 288 points of damage . . . assuming all six rays
hit their target.