As Dee was saying, light falls off over distance. So if you have two lights aimed at your subject, and the backdrop is behind the subject, then the backdrop is getting less light. This is especially important if you are trying for a white background, because even a white backdrop, will look grey if it gets less light than your subject (assuming that your exposure is correct for the subject).
This is why most studio lighting scenarios, include at least one separate light on the background only.
Also, when learning to use studio lighting, don't just place one light on either side of the subject. Instead, concentrate on just one light to the side. Use that light to get some creative light and shadow on your subject. The 2nd light should be very close to the camera position and it's job is to 'fill' in the shadows...but when you're learning, just try one light as first.