This might work..
Create a second account on the device (2nd Android account) and a 2nd profile in Netflix
Install Netflix on the 2nd Android account
On the 1st (main) Android account, get your movie of choice going and then reboot your phone so that it is no longer connected
When the phone reboots, log into the 2nd Android account, open Netflix, connect to the other Chromecast and see if that works. You'd be oln a separate Android profile, separate Netflix profile, separate Chromecast. It might work