What is the point? As both lanes converge at the same window, the McD's is going to end up servicing the same number of people that it would have serviced using one lane. The second lane only adds confusion, frustration, and the possibility of incident to the drive-thru equation. Of course, there's the argument: "But the McDonald's would be able to process twice as many orders." Bullshit.
Let's say McDonald's WAS able to process twice as many orders. Those orders would just end up bottlenecked at the window, as you would have two orders ready for every one car at the window. Moreover, if one gargantuan, complex, "I-have-four-morbidly-obese-teenagers-in-the-back-of-my-minivan" order preceded four easy "fries-and-a-Coke" orders, you'd have a backlog of lower-quality, soggy fries and watered-down Cokes. Then, you would have lower customer satisfaction, then you would have less business, then you would have to dismantle and sell off your expensive, second drive-thru intercom. It's a hideous, downward business spiral, but I haven't mentioned the worst part yet: the inevitable injustice of assigning car priority at the order intercoms.
True story: I stopped at the aforementioned McDonald's 10 days ago. As I turned the corner at the rear of the building, I noticed the new, dual-drive-thru-lanes scheme. Seeing a motorist backing out of the outside lane and stopping to talk on his cell phone, I opted for the inside lane. A motorist was already at the inside intercom placing his order. The outside lane remained empty. The motorist in front of me (let's call him Gil), finished placing his order, but could not pull forward, as a car was in front of him. The outside lane remained empty. The car in front of Gil pulled forward, but Gil, probably a green light sloth, sat there adjusting his crotch for 3 seconds, allowing just enough time for a car to pull up to the outside lane intercom (this was not the original, cell phone car). While I was shouting obscenities at Gil, Gil finally got his junk in order and pulled forward. I arrived at the inside lane intercom, and got a "just a moment please" from the speaker. I turned to my right and saw the guy at the outside lane intercom finish his order & start to pull forward to a position behind Gil.
Ready to go off like a neutron bomb, I instantly boycotted the McDonald's, threw Rusty (my car) into reverse, and sped off to the adjacent Stop & Shop for muffins. You lose, McDonald's. You might as well put that outside intercom on Craigslist now, 'cause Stop & Shop makes a mean Raisin Bran muffin.