If you gently pull the 1st film's tape from the backing paper and fold it over onto the film itself and then wind it right round to the centre of the spiral, the second 120 will not be able to pass if it should get to the contact point, the groove will be too narrow. No problem using the usual amount of dev as far as I have found. I frequently dev 2 x 120's together using this method. No probs with loading or dev.