Originally Posted by Rafal Lukawiecki
I would be skeptical of this too. Let's look at single-bath fixing without a wash aid: Ilford and others give a limit of 2g/l of dissolved silver compounds for a "commercial" or "general purpose" level of permanence. For "optimum permanence" a much lower level of 0.5g/l of silver in solution is given, which reduces the capacity of the single fixing bath to about 10 8x10s per liter.
If you want this level of permanence (and any exhibiting photographer should strive for this IMO), then you should toss a single fixing bath after a rather small throughput. Adding a sodium sulfite/wash aid to this single-fixing bath is simply not going to increase the capacity by 4x. From what we've been discussing, it may (emphasis on the "may") increase the fixer capacity a small amount, maybe even 10%, but likely less. This would allow you one extra 8x10 from your single fixing bath. You would have to test print 11, and 12, 13 ... if you really want to find the limit of this method every time for many runs to come up with some kind of empirical idea of what the capacity of single-bath fix plus wash-aid bath as fixer (or even a more concentrated sulfite bath) was. As I've mentioned before, this has likely been done by the chemists at Kodak already. If it were a promising path, I believe it would have been explored more already. Since it hasn't, I would tentatively conclude that it's simply not worth it to use a sodium sulfite bath as a "fixer extender" or a second fixing bath.
With two-bath fixing, as Gerald so adroitly points out, you have, in essence, a fresh fixing bath in bath 2, which should never reach even the 0.5g/l level of dissolved silver if used correctly. This reliably ensures that complex argentothiosulfate compounds are reduced to more soluble ones which will wash out. Easy, proven, reliable and you don't have to do any time-consuming tests.
Whatever method of fixing you finally decide on, just make sure not to exceed the capacity and test often. I test the last print of my fixer run for residual thiosulfates and residual silver each toning session.