You'll probably find that in order to do this, you'll have to mirror the refresh rate and resolution on both. To do this, you'll have to set the display settings to match the TV... so you'll end up with a comparatively low refresh rate and res on your monitor. Not easy on the eye.
However, you can easuly switch from sending the signal to your monitor to your TV and vice versa. So there's really no need to mirror the screen accross both displays.