I suppose you could rule that those spells that don't have a saving throw would have a commensurate penalty to the attack roll (a lot of spells that don't allow a saving throw require a touch attack).
I can't remember the rules for Harm (does it require an attack roll?) but that still leaves a problem with magic missile. Maybe duel wielding a wand of MM could result in a damage penalty?
Or maybe dual wielding could warrant a (reasonably high)Spellcraft check. 15 + spell level to get the spell off (if you fail a charge is wasted)? Followed by a Wisdom check (DC 5) to avoid a mishap, as if you were casting a spell from a scroll that was too high level.
I don't know if that sounds overly harsh. But I think that if activating a wand is like turning a mental key in a lock, then activating two in 6 seconds should be hard to do. Even with the appropriate feat.
Dual Wand Fighting
Prerequisites: Combat Casting, Spellcaster 5th+, Spellcraft skill
Benefit: As a full round action, you may activate two wands (one in each hand). When using this feat, both wands require a Spellcraft check (15 + spell level) to activate, or the charge is wasted. If you fail to activate the wand correctly, you must succeed at a Wisdom check (DC 5) to avoid a mishap.
Normal: You may only use one wand at a time.