If you can place one left and right of you then that would give you a good covering when you turn away from one....but you might be best getting a extension cable for one and putting it behind you, amazon basic 6 feet USB 3 ones are fine, i have had two running for 3 years now with no issues. 😉
If you go to this link................
.......... you'll find a detailed Oculus 2 sensor, 360 degree set up guide.
It gives a lot of info about the one in front and one behind setup.
To answer your question about sensors on your desk. If the sensors are on the desk, and your play area is close to the desk, then part of your play area will be in the "shadow" of your desk. Better set ups have the sensors mounted high up to reduce the "shadow".
I have 3 sensors. Front Rt is direct connection, Front Lt has a basic unpowered 2 metre extension and the rear has a 5 metre active extension. All sensors have worked fine for 3 years now.