If you double lunged and saw it go down, I would shoot the 2nd also.
I had the opportunity once. Shot an eight point and nocked 2nd arrow to have a nice six walk out of the corn in front of me. I didn't shoot because I did not know yet that the first one was down. I could still see the corn moving where he was down thrashing around. I did not want crossing blood trails. Had I known that he was indeed down when the six walked in front of me, he would have gotten some thunder too!

Keep your knife sharp!