ImgBurn will give you a list of possible layer break positions. Most will be very close to 50% of the data being burned. There is no guarantee that it will split exactly between the two files.
The layer break is just where data being burned to the disk goes from the first dye layer of the disk to the second dye layer. Unless these are dvd-video, you shouldn't have a problem with position of the layer break. DVD-video has to have the layer break positioned on a cell boundary and the cell must be on the edge of an ECC block