Best to work in whatever version they were created in.
As far as I know CC includes InDesign Cs6.
You can install the application on two computers, as long as you're not using the application on both computers at the same time.
If you can work in CS6 then great. If you've opened the file in CC and made edits you're happy with, you should install CC on the other computer.
If you want to go back to CS6 then in CC File>Save As>IDML and open that file in CS6.
Install CS6 on the other computer so you can work on both computers in that format.
Download InDesign CS6 from CC
Found the install for cc, so going to download it to other pc.
As document saved in cc. Thanks
I downloaded a second user now onto another pc. It downloaded CC as only option to download.
I'm not sure what you mean by this. Did you buy a second subscrition, or are you using one subscription on two computers? The latter is allowed, but ONLY FOR A SINGLE USER who does not use both at the same time. If two people are working simultaneously you need two subscriptions.