Tetenal for example, states that you can reuse the working solutions 3 times with increased times. On the other hand they state that if you want highest quality and consistency, you should use the chemistry one-shot.
Please note that they issued a completely revised version of the Colortec E-6 manual a few months ago where they clarified some issues that were not explained very well in the old manuals.
For all working solution volumes (250/330/500/660/750/1000ml) they give the times for 3 consecutive runs.
At 250ml, which is the volume i use for rotation in my Jobo 2520 tank, they additionally give the time for 2 films at once, which uses the times of the second run. Probably to take the gradual chemistry exhaustion into account that happens during
processing and which is higher when processing 2 films at once.
However, this special case (250ml and 2 films at once) is obviously intended to be one-shot as they don't mention any further runs.
They also explicitly mention the possibility of using a 2min prewet (with 38°C water) to bring the film, spiral and tank to temperature and to avoid uneven development. Development times do NOT have to be increased when using prewet according to
the manual and my personal experience. I use two 1min prewets because at the first fill, the water gives off its temperature to the tank and spiral and then it is some tenths below process temperature.
As i said earlier, i switched completely to one-shot development and try to develop 2 films at once if possible. For some time, i used longer FD times for Velvia 50 but now i rate it at 40 or 32ISO and use the same 6:15min as for the other films so that i
can mix Velvia 50 with other types of Film.
When i did 2 or even 3 runs, i experienced problems like films that where too dark or had color shifts. Especially with run number 3 but also with the second. I think that problem is further amplified by the fact that i use only 250ml of solution. Maybe some
reaction side-products accumulate and influence the process more when the working solution volume is lower, i dont't know. I just had results in the second run that where not as good as i expected them to be.
Another possibility could be the additional FD dilution from the residual water left in the tank due to the pre-wet.
A telltale sign for me was the tint and density of the film marking/frame numbering. With that i could tell at an instant that there was something off in the second or third run while the first one was always spot on.
Anyhow: split concentrates, inert gas, 250ml working solution, one-shot with either 1 or 2 films and then dispose, that's what i do and it works flawlessly for me. I have not damaged any roll of film since i do it that way.
But i would be happy if Tetenal would offer the FD concentrate separately because it is usually the component that goes off, if such things happen. As i do not reuse chemistry anymore, i go through the whole pack in a reasonable time and i seldom have
any "duds" among my small bottles. And if there is a discoloration, i toss it because i do not hike the mountains and walk the fields for days only to ruin my films because i wanted to save a few bucks