This extension allows you to select which device each YouTube video plays to by adding a drop-down right above the video description. Devices are stored per-session, so the device will not reset when the next video is played via auto-play or a playlist.
This extension is targeted toward streamers who need to play videos into a specific device. However it is also great for listening to music on your speakers and your headphones at the same time! (I have no idea why’d you do that, but you do you man, you do you…)