They're not the flatest I've seen. Do you have a flash meter? If so meter the heads seperately and try for about a stop's difference. You can also adjust the angle of the lights, if they're both very front on it will not give you a decent differential.
As another option, grab a big sheet of polystyrene and use it as a reflector with just one light, will set you back only a few quid. Paint one side of it black and you have a big subtractor too.