Quote Originally Posted by mts View Post
...Fixing is either complete or not...
The problem is, fixing is never entirely complete. To decide on two-bath over one-bath fixing depends on how complete you want it.

During fixing, undeveloped non-image silver is dissolved by thiosulfate without damaging the metallic silver image. The first fixing bath does most of the work but becomes increasingly contaminated by the soluble silver thiosulfate and its complexes. Soon, the entire chain of complex chemical reactions cannot be completed successfully, and the capacity limit of the first fixing bath is reached. A fresh second bath ensures that all remaining silver halides and silver thiosulfate complexes are dissolved. An intermediate rinse is optional, but it protects the second bath from contamination. Fixing time must be long enough to render all residual silver halides soluble, but not so long as to allow the fixer and its by-products to permeate the paper fibers; the former being far more important than the latter.

So, one-bath fixing works, but it has some disadvantages: longer fixing times, fixing never as complete as two-bath fixing, residual fixer penetration deep into the fibers, leading to longer and not as effective print washing.

Quote Originally Posted by mts View Post
...If you are not re-using fixer you are unlikely to exhaust the one bath...
Correct, but if you do not change the fixer after every print, you are reusing fixer.