If indeed you wish to dual boot you would need to buy a new copy of Windows 10. I am lead to believe that yes you can download from Microsoft as a separately paid for file and then burn that as an ISO to disk or thumb drive. Microsoft are also seemingly trying to get away from buying on disk and are offering it as an ISO file on a thumb or flash drive.

However, there is a long and complex way of rolling back from Windows 10 to your previous operating system if not happy with Windows 10. providing you do it within 30 days from installing Windows 10. I will also warn it is best to create a full backup of all your data before either upgrading to Windows 10, or rolling back to the previous system. more here :-

https://techingiteasy.wordpress.com/2015/03/24/how-to-rollback-to-a-previous-version-of-windows-from-windows-10/