Hey there BB42,
Welcome to the Chromecast Community Forum! Currently, you can only cast to one device at a time. If this is something you'd like to be able to do in the future, would you be able to submit feedback via Google TV? Google takes all feedback seriously, and the more feedback they receive regarding a specific feature, the more likely they are to prioritize it.
I look forward to your response, and if you have any other questions or concerns for me, please let me know! Otherwise, I'll lock the thread in 24 hours.