Just like the title says. The warlock invocation eldritch glaive lets the warlock attack with the glaive effect "as if wielding a reach weapon." Is this sufficient to let a warlock use two-weapon fighting in conjunction with the eldritch glaive? (I know that if it's allowed, then the warlock'd probably have to use in his off-"hand" unarmed strikes or armor spikes.)
A warlock must take a unique full-round action to make attacks with an eldritch glaive (Dragon Magic 82). The invocation's description, in part, says, "As a full-round action, you can make a single melee touch attack as if wielding a reach weapon."
While the Player's Handbook on Two-weapon Fighting, in part, says, "If you wield a second weapon in your off hand, you can get one extra attack per round with that weapon" (160), earlier on Full Attack, in part, says, "If you get more than one attack per round… because you fight with two weapons…, you must use a full-round action to get your additional attacks" (143). (Bear in mind that the section is titled Full Attack; it's not like the run action—also a full-round action action—entitles a creature to make extra attacks because the creature's wielding two weapons; it's the full attack action specifically that allows a creature to make extra attacks because the creature's wielding two weapons.)
Thus the full-round action that eldritch glaive requires isn't just, like, code for a full attack during which the warlock can, for example, employ two-weapon fighting, but, instead, the full-round action that eldritch glave requires is its own special full-round action and, because of that, it's sometimes sadly incompatible with other combat options… like two-weapons fighting.
…But ask the DM
It's probably unlikely to create problems if a DM allows anyway eldritch glaive and two-weapon fighting. Two-weapon fighting is always resource intensive if it's to matter, so devoting feats, gp, ability score increases, and other resources to improving two-weapon-fighting-while-eldritch-glaiving likely won't raise any balance concerns: adding to his ability to two-weapon fight means subtracting from his ability to do cool warlock stuff… and, ultimately, that other cool warlock stuff probably would've been more optimal than two-weapon fighting.
The only potential wrinkle I can imagine to ruling that eldritch glaive can be used with two-weapon fighting is that folks might argue to extend that ruling throughout the game so that two-weapon fighting extra attacks can be made during any full-round action that involves an attack—like a full-round action Tome of Battle-style martial strike or maybe even a mundane charge!
That… really doesn't sound terrible, to be honest, but such a change may be a concern for the DM if the DM is worried about radically altering the game.