Hey there Ern,
I apologize about the late response, thank you for being patient! Were you able to figure out that setup? If not, I'd be happy to help out, as I know these issues can get tricky. However, I'm not too sure I understand what you're asking for. Do you want to set up the Chromecast on the TV downstairs and upstairs? Or do you want to control the Chromecast downstairs from the TV upstairs? If you want the Chromecast on both TVs, you would need two separate Chromecast devices.
I look forward to your response, and if you have any other questions or concerns for me, please let me know!