I recently tried to use 2 lights -- one elinchome 600RX to create a pure white background and a main flash with my speedlight. Both work nicely individually, but when I combine both the Elinchrome studio light overpowers the Speedlight no matter what settings I use. Normal settings wihtout the studio light gets me nice lighting, but the exact settings show an underexposed subject with both lights. The difference then went down to changing ISO.
I'm using a Canon 20D, shooting on Manual Mode.