This website allows you to view multiple (up to 9) YouTube livestreams, along with their respective live chats, all in one page.
To use it, visit https://multiviewer.escfent.com/?streams=, followed by a comma-separated list of the streams' YouTube video IDs.
The video embed system is itself an embed of VidGrid, a great stream multiviewer website: https://vidgrid.tk.gg/