How do we avoid the frontal flash creates a halo like shadow when we shoot with flash

Hi, I think everyone knows that we can use the pop up flash to common mutiple slave flash to create a good lighting set up. I however found it disturbing to have the frontal flash light up the subject.

Is there a TTL wireless triggle I can use to make one of slave flash become the common flash?

I have three SB900 and a D90 and a D700.