Formal Portrait Lighting

I shoot weddings and am trying to find the best combination of light, diffusion and portability for when I take large group formal portraits. I have two 580EX's and an ST-E2 which will fire them remotely when positioned to my left and right but I feel the shadows are too harsh. I havn't tried umbrellas yet which I think will help a lot. I've put a variety of diffusers on which help but I feel I need to take this to a new level. I need suggestions to pull this off without too much gear and setup. Any recommendations?

Umbrellas would be your best bet to soften the light. Or you can try bouncing your flash, although that is not always an option.

Could you post an example pic?

With a light left AND right you shouldn't have too much trouble with shadows.
Even without a diffuser.