Don't think of the background light as just a background light. Play with it.
If you double the distance you will diminish the fall-off.
You will need to double the output, and probably put a card so you don't spill light to your subject.
But play with the light, cut a cardboard in different shapes.
From different angles
Put a diffuser before the cardboard, remove the diffuser, move the distance to the screen, to the cardboard, the angle.
You can also construct a snoot just wrapping cardboard around your flash head.
Take a time to shoot not your portrait, but to shoot just backgrounds.
After you play with background's illumination probably you need to separate a bit more the subject from the background, because the light of the main subject will contaminate the background. Again, play with the distances.
I'm adding this part. Lighting the background will not add "separation" on its own. Playing with the illumination on the background will give you an ambient, hopefully it will add some mood or a story to your portrait.
But to "separate it" you need to play with the illumination between your model and the background. A hair light; a shadow-light vs light-shadow scheme perhaps; or narrower DOF.