Well, that is not exactly accurate. The person you are writing to is not a "business," the person you are writing to is an individual. If you hit that individual with a request to submit two scripts, well people are hesitant to take just one script from an unknown source because it might be really bad. And someone for sure is not going to want to get two scripts thrown at him or her until he or she knows you can write. So asking to submit two scripts is right off a red flag and makes someone less likely to say yes. And --
If you say, Hey, I'd like to send this script to you and here is what it is about, but hey if you do not like that one how about this one? It sounds like you are not committed to the first project you pitched. Which again, makes someone hesitant to take a leap of faith and ask for the script because if you do not believe it can stand alone, why should he or she?
Once someone has read a script of yours, if he or she thinks you can write, he or she will ask what else you have got. Unloading everything you have got out of the gate however is not always the best way to go.
This is of course my opinion. Mileage may vary.