there is no reason for the 2 anode, 1cathode. You could add as many as you want, the cathodesurface is the most important.
Like said in my previous posts, I use Arc-Air carbon rods, their surface is certainly much smaller than my cathodesurface, I doesn't matter.
they seem to have enough surface to carry the current (I'm only putting 1 amp through them, so they are ok)
Search for a rotating drum recovery unit patent online, the cathode surface is always much bigger than the anodes they use.
if your flow is straight on the sharp edge of all cathodes, i don't think there would be any current-difference issue. If there is enough flow, the difference in concentration of silver would only be marginal.
If you are trying to re-use the fixer, there are some issues to consider.
Don't go too low on silvercontent, because depositing sulfide is altering the chemical composition of the fixer, making it useless for re-use.
watch for changes in pH, watch temperature...
I think in one of the Kodak publications there is some mentioning of reusing fixer, you would certainly have to add something, or replenishing with fresh fixer.
I have reused some of the depleted fixer, to get the silver of undeveloped (expired) film, so it would be re-usable. have not tested this with real x-ray for commercial NDT work, I think there might be quality issues with re-used fixer. Be careful selling the fixer back, they might claim you when their client are not accepting the radiographs.
regards
Fonnie