advice for light setup - indoor portrait

Hi all,

I am going to shoot some profile pictures of work colleagues to be used in Outlook, Lync, LinkedIn etc. I think it will be a portrait that includes shoulders or half chest (not sure what the technical term for that is)

I am unsure of the setup: from what I learnt (strobist) it is better to have natural light as fill light coming from behind the camera, and use an umbrella as key light; work with shutter speed to get enough natural light to do the right fill.

The problem I have is that I don't have a backdrop to place opposite of the window. Instead I have a white wall 90 degrees to the window (the windows are half size of those below). I have thought about the setup below but I'm afraid of shadows created by the two light sources hitting the subject 45 degrees

Also the window is facing south and the wall east, and I'm in the southern hemisphere so those windows never see the sun (if that is relevant)

At the moment I only have one off camera flash (430EX) and a white umbrella