how and where do i download a copy of photoshop CC on a second computer ?

You will install it the same way you did on the original computer.

Creative Cloud Help / Creative Cloud for desktop

https://helpx.adobe.com/creative-cloud/help/creative-cloud-desktop.html

Creative Cloud Help / Sign out, Sign in | Creative Cloud desktop app

http://helpx.adobe.com/creative-cloud/kb/sign-in-out-creative-cloud-desktop-app.html

Creative Cloud Help / Install, update, or uninstall apps

http://helpx.adobe.com/creative-cloud/help/install-apps.html

Installing Creative Cloud Apps

http://tv.adobe.com/watch/cs6-creative-cloud-feature-tour-for-video/installing-desktop-apps-from-creative-cloud/