Can Heighten Spell and Eldritch Corruption stack?
Ask the DM
The benefit of the feat Eldritch Corruption says, in part, that
You can enlarge, extend, heighten, or widen a spell, as though you had the appropriate metamagic feat, without increasing the spell level or, for spontaneous casters, casting time. You can apply any number of these metamagic effects to a spell in this way; if you choose heighten, then that spell’s level is heightened by up to two levels. (Heroes of Horror 123)
So while the Eldritch Corruption feat's benefit is as though you had used the appropriate metamagic feats, you haven't actually used the appropriate metamagic feats. This sort of language is used throughout D&D 3.5, and the game never definitively states that when something is just like something else then it actually is that something else. So whether you can combine something and something else that's just like something only called a different name is up in the air, which is dumb and frustrating and why you'll have to ask the DM.
This DM would allow it
Given the restrictions on the feat's acquisition from the feat's type (it's a tainted feat) and its prerequisites (taking the feat requires the creature be (at least? only?) moderately depraved, the effects of which are difficult to game—but the depravity treacherous is, at least, hilarious), increasing a spell by 2 levels by dealing an ally 2 points of Constitution damage seems reasonable even if the feat Heighten Spell has already been applied to the spell.