I second Bertil's words and would like to add some of my own trickery.
Uneven development along the edges of 120 film is a tricky issue that I'm not sure I understand fully myself. Everybody seems to have their own recipe for success, and I am no different. Sometimes I fail as well, though only rarely now. My road to success:
1. agitate at least the first 30 seconds (I do 45),
2. make sure the tank is big enough to allow all developer to clear the film when the tank is fully inverted,
3. use a bit more solution than what is indicated on the tank (e.g. 600 ml instead of 500 ml); this helps against air trapped under the top reel,
4. tap the tank *lightly* after agitation (but first allow some time for the solution to settle, otherwise the film may kink) in order to release air bubbles that cling to the film,
5. don't just invert the tank, also rotate (combine the two),
6. avoid short developing times (<5 minutes).
Two 120 films in a 1 litre tank is a no-no for me also, even with the large-head Patersons. I speak from experience .
Originally Posted by Bertil
The coca-cola bottle solution against shifting of the reel up the column may be substituted by adding an empty reel on top of the reel containing the film. Or use the retaining rings Paterson used to supply with the older non-Super System 4 tanks (I don't know why they stopped including them in the Super 4 system- shame).