The little tab works great for me for last 10 years. I almost never do a single 120 on a reel; always two. Same with my 16mm reels, I usually put two 18 exposure films on a single reel and use the little red tab.
Always make sure the first roll is all the way or, obviously, the rolls will overlap.
How do you know? When you have the first rool loaded, there will be some 'springyness' to the end of the rool; it will be mobile in-and-out of the reel. If you inadvertently put the red pin down ON the edge of the first roll, you wedge the film in place and that mobility of the end of the film will be lost. This is easy to feel for. If this happens, just release the red pin and wind the first roll ALL the way on the reel this time. If the first roll won't go ALL the way in the reel...well, that is a 'reel loading' problem and you have to use all the little tricks to get it to go all the way on.