Lighting Set up doing a promo shoot...

I just got my hands on the elinchrom ranger quadra kit with two heads, at the moment they both have 13.5cm reflectors which causes fairly harsh light.

I'm doing a promo shoot in two weeks time and I'm fairly new to lighting, so im pretty nervous about it, the shoot will be held outside in the day light but will have a grity and dark looking surrounding. The models will both have there backs back to back. Would it be wise in buying a 18cm reflector and a 70x70 softbox?

Softbox would be facing down onto there heads which would be postioned just above the camera and the 18cm reflector would be under the camera facing up.

Any ideas would be much appreciated how you would do the shoot and what type of lighting equipment would you use?

If they are going to be back to back I would want to use both heads and have equally soft light on either side. Lighting directly above the camera will be flat in my opinion and not mix well with the gritty surrounding.