Yes, you can stream to multiple platforms at the same time after you add their Stream Key and Stream URL to your Studio's Live Stream settings. In the settings section, click the toggle to enable each platform you want to include for your live stream.
Viewers watching on other platforms do not your Riverside Studio's sidebar chat.
To see any chat or comments posted by viewers on other platforms, open the stream's page/s in another browser window or on another device. Be sure to mute the streaming video.