When it comes to lighting a white "infinite" background style talking head, I usually forego the backlight entirely since it tends to make the outer edge of your subject appear to blend in with the background. This gives the edge of your subject a "soft" or glowing effect that is best avoided. However, if the backlight is dim enough, you may be able to still get away with giving your subject a bit of light on the shoulders and hair. The best tip is to try it first and see what you think. You never know what you're missing until you've given it a shot.
Also, if you're doing a full body shot, I would definitely light your subject with a soft-box or heavily diffused lights (through a scrim or bounced off of a white object). This way you won't see any noticeable shadows at your subject's feet or a background shadow from their body, allowing you to fool the audience into thinking that it's a true "infinite" white background 🙂
Hopefully that helps a bit! I'm sure others will have plenty of great tips as well!