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You can set up as many Chromecast on the same WiFi. However, you must select different names for each device. It is also a good idea to ensure that your WiFi network is password protected. When you introduce a new Chromecast to your network, simply repeat the instruction when you first initiated your Chromecast. One thing to be careful of is to ensure you choose names which is easy to remember. As you cast the screen from the content you are watching, all available devices will be visible when choosing a Chromecast. Also note there is no way to block someone from casting content to someone’s screen. In other words, if your sister is using Chromecast to cast something from her iPhone’s Youtube app to her TV, you can accidentally stop her cast and put something to her TV through your Android phone.