No, CoScreen works best if you have two or more displays because this enables you to share windows intuitively by drag and drop from your private display to your shared display and you can separate the two cleanly.
But it also works great with only one display, e.g. if you're on the go. You're able to share either all windows on your display, none, or in future also individual ones.
So one or the other way, you're always in control what you share, no matter if you have one or multiple displays.