It depends for me. For C-41 I try to maximize the number of films I run with the 1L of fresh chemistry, and then maximize any second run done within a day of the chems first being used.
With Pterson reels, I am able to load 2-120 rolls end to end on the same reel, joining them with film splicing tape (blue max brand) after the first one is approaching being fully drawn into the spirals.
Then I can usallly feed 4-120 rolls though 1L of chems.
With my stainless tank I have never been able to sucessfully double load rolls end to end.
I have never been able to convince myself that loading films back to back is a good idea, but have heard of others doing this. If I did, then I would stick to stainless.
I like Paterson SS4 tanks, since the chems pour in and out faster than with my stainless steel tanks. I usually resort to taking the lids off in the dark when doing c-41 in stainless to get the developer in and out consistently enough at just 3:15 for the processing time. Then the challenge is to find the lid when it comes time to agitiate the stop bath.