Load the first film. Once it is past the ball bearings, place a thumb on the end of the film, and while continuing to walk the reel back and forth, you can feel the film continuing down the spiral. A very gentle pressure may be needed to push the thing along.
At some point it will not spiral in any more, and you will fiddle for a second or two before you are sure this is the case. This is normal for me, even after doing this for quite a while.
I then begin to load the second 120 film. I usually take the backing paper off first, but be sure to do so for this second roll. I hook a pinky finger of my 'still' hand into the film to keep it from getting too close to the guide path and getting stuck. When I feel the roll to be loaded disappearing, I slow down on the reel rocking, and stop just after I feel the film get onto the spirals.
I would recommend pratciting with scrap rolls first, but it does reliably work for me.