Q: Dear Sage
Can a warlock use Rapid Shot to fire two eldritch blasts simultaneously?
A: No. Using eldritch blast requires a standard action, not an attack action (unlike using a weapon).
If something requires a standard action (as opposed to an attack action) to use, you can’t use the full attack action to gain extra uses of that ability, even with the Rapid Shot feat.
About the Author
The Sage is a mammal classified in the bear family, Ursidae, native to central and southern China. Debate continues as to whether he is truly a bear or more related to the raccoon rather than the bear, or perhaps in a family of his own. When not eating bamboo and answering rules questions, he works under the nom de plume of Andy Collins as an RPG developer in Wizards of the Coast R&D, where his development credits include Complete Mage, Dragon Magic, and the Player's Handbook v.3.5. He can be easily recognized by his large, distinctive black patches around the eyes, ears and on his rotund body.