You have a major misconception. Fixing doesn't darken negative film, developing does that. Fixing for longer will not make thin negatives darker.

The developer turns the exposed light-sensitive compound into something dark. The fixer removes the remaining unexposed light-sensitive compound so that it won't darken over time (it eventually does that even without being developed). Once all the unexposed material has been removed, additional fixing won't change the appearance.

Most of the fixing happens in the first few seconds or tens of seconds, with the remaining time making sure the last bit of unexposed material is really gone. Putting film back in developer after having been briefly exposed to fixer might sometimes yield slightly higher density. However, this could be splotchy due to the different amounts of time different parts of the film were exposed to the fixer as it was poured into the tank. Anything much longer than the pour time is unlikely to yield much of a result.

Basically, once the film touches the fixer, consider the developing done. In your case, it was absolutely done after the initial 20 minute fixing bath. There is nothing more you can do. Any remaining underdeveloped silver-halide is long gone. No amount of developing is going to bring it back. And certainly more fixing won't do anything useful. Eventually the hardener in the fixer will make your film too brittle, but you really have to overdo it for that to happen.