Why would i install two windows 7 OS together?

This is the answer to the question which one person asked me in the past week.
Question was like thought before the 2 years!
If you want to run applications etc you can run on one then why do you need the other?
 but quite surprisingly asked me that have i ever tried to install Mac OS ?
I had no words to say!
 Then they shifted that have i ever tried to get the two OS together?
 My answer was incomplete.
If one file is corrupt and other is in good postion then why i would use the corrupt one.
 I meant that i would not try to get the file which have virus in it and corrupted Especially already in use!!!

The time was not so much that i could explain them. But they simply said they knew everything. I asked what they knew?

 They are like a file which don't talk so much!
 But i want to say that is not a file , there is virus in it.
 and i ANTIVIRUS know how to treat with that virus.
If we don't know anything completely we must not even say something!
 Otherwise i can't have Mac directly but can install Mac OS theme and can show u MAC.
But the main thing is to remove the virus from your all mind! Here is MAC, OS , FILE is indirectly saying something!