It would help if you could clarify what you have set up so far: Am I right in thinking that you are pointing the two heads from the kit you bought at the back wall of the Lastolite using the side vents (i.e. one on each side)? And what are you using to light your subject in front of the Lastolite exactly?
One head in the right hand side vent of hilite (half way up - 45 degrees to the right so pointing to the back)
Other head on subject in the form of softbox.