A couple of months ago in Dragon the Sage ruled that with the exception of Empower + Maximize (which is already clearly defined) that metamagic feats stack multiplicatively.
Ex: A Twinned Widened Maximized fireball would result in two 40' radius fireballs each doing 60pts of damage.
Ex: A Twinned Widened Maximized fireball would result in two 40' radius fireballs each doing 60pts of damage.