took me a while to figure out how to do it. So first off, I loaded the dress and added Fit Control to it. then I loaded the dress a second time. I lowered the opacity of the second dress to make it more transparent. Then using Fit control, I made the first dress too small in areas using sliders, like chest, hips etc. this then makes those areas of the first dress sink below the skin, but leaves the second see thru dress visible. It takes alot of messing around with the sliders to get what you want, and not make the two dresses get into different positions.
Hope that makes sense. I have been messing around trying to get this right for awhile now, off and on. but finally think I have it.