It is possible to have the same file in two folders in Google Drive. What I don't know is why the feature is hidden. You cannot find it at a glance. This guide will help you in case you have a file that you need in multiple folders but you don't want to maintain 2 different copies of the same file.
Here it is what you have to do:
1. Select the file that you want to have in another folder also.
2. Press keys: Shift + Z (The menu in the image will appear)
3. Select the second folder where you want the file to be.
By doing this you will be able to open the same file from both location and cooperatively work at it with your coworkers.
Note: The file exists in the same time in both folders. So if you delete it from one folder it will be deleted from both.
To remove the file only from one location, but keep it in the second you need to do:
1. Right-Click on the file;
2. Select View Details;
3. Select 'Details' tab from the right side;
4. In the 'Location' select the X icon for the folder where you don't want the file.