Depends on the the age and model of the tank. Some came with little clips that pushed into special slots in the reel to divide the films, and stop them overlapping each other. Others were never designed to take two lengths of 120. Two 120 films are slightly longer than 1 35mm, so the reels may not take the extra film at all.(posted 8789 days ago)
Assuming you can fit the 2 films in. The easiest way is to use a piece of masking tape to stick the two films together, and prevent them overlapping.
If you're going to be doing this a lot, get a double stainless steel tank and a couple of reels. Much easier in the long run. No sticking or buckling of the film, heat dried in seconds, less solution per film, quicker pouring......have I forgotten anything?