How can I double a song's length?
Well you can use any digital audio editor to combine songs. Like Audacity. 


  Audacity is a free open source digital audio editor and recording
  computer software application, available for Windows, Mac OS X, Linux
  and other operating systems.


Here is link that tells step by step how can you do that using Audacity.

http://www.wikihow.com/Combine-Songs-on-Your-Computer-Using-Audacity