My random thought of the day was that drive-thrus with two lanes that merge into one need to have some sort of control for the merge to prevent idiots from jumping in front of you when they ordered after you. I can't imagine the two lane system is really any more efficient, other than maybe kind of shortening the total length of the line around the building.
I'm no fast food magnate...I just hate the merge lol
We have a two lane drive-thrus here too. They are quicker, provided there are two staff taking orders. Two orders taken at once means both can be commenced at the same time, which means they are made sooner, which results in less time each vehicle is waiting at the collection window. I don't know the math on how much more efficient it is than a single lane, but enough to make it worth while. Also, big queues dissuade some potential customers, resulting in lost sales, which of course you want to avoid. More cars attending your drive-thru in less time equals more money.
As for the merge issue, well some people are just jerks. Could have some sort of gate or light system I suppose, but the businesses would probably see it as not worth the cost.