If you have an incident meter then determine exposure by placing it on the white background and aimed at the camera. After this shot recock the shutter and fire it with a lenscap over the lens. Recock the shutter again and you are ready for the second exposure of the in-camera masking set. This should have moved the transparency out of the way and rolled it up onto the take-up spool. Now to make your second exposure, replace the white background with an absolutely black background behind your subject.
Black velvet is best.
Move your lights so that they illuminate your subject as desired but without spilling much light onto the background. Without changing the aperture, determine the exposure time required to properly expose you subject using an incident or reflected meter reading but make sure to make it based on using the film's normal speed rating. Since the raw film stock was not connected to the sprocket drive of the camera it should not have moved between exposures. If it was connected to the supply spool then you could at this time rewind the film into the supply cassette and process it. Otherwise, empty the camera in a darkroom and complete the development process.