This question was recently asked by Thibault:
“I sent a few requests in Paris for a weekend and I’m in conversation with two people who might accept me. What happens if one person approves the exchange before the other? Does the other request get automatically cancelled?”
That’s a very valid question: are the members you contacted automatically informed that your request is no longer relevant?
The answer is no: at this stage, the people you contacted still believe that your message is up to date. They might even be discussing it with their family or friends before getting back to you.
That’s why it’s important that you send them a quick message to let them know your request is no longer active, since you’ve already found a match in the meantime. As always, try to put yourself in the other person’s shoes: you’d also like to be informed in such a situation, wouldn’t you? 
As we always say, communication between members is the key to a strong and harmonious community… Happy exchanging! 