Just for fun (I had time on my hands), I made a crude spreadsheet model to compare how much paper could be processed in three configurations. They are single tank fixing and two-tank fixing (both with and without replenisher).
I made various assumptions regarding how much solution carry-over there is, how much silver is in the paper, etc., etc., so the results are not necessarily valid elsewhere. Consequently, I won't report the actual calculated capacity, but rather just compare the three process configurations.
I set the capacity limit where the final fix tank got to 1/2 gram/liter of silver.
A) Single tank, 1 liter, fixes one baseline unit of paper.
B) Two-stage fix, NO replenisher (1/2 liter in each tank = 1 liter)
- fixes 2.5 baseline units of paper.
C) Two-stage fix, WITH replenisher (based on steady-state replen rate)
- fixes 4.0 baseline units of paper (per liter of replenisher)
note: I'm underrating case "B" since the "exhausted" fix-2 still has more capacity in the fix-1 position. I just don't know a graceful way to put numbers to it.