The part "A" tends to get absorbed by the film and goes away at the rate of about an ounce per roll. eventually you will have a full container of "B" and very little "A" left. At that point I dump both and mix new.

Any fixer should be fine, I use both Kodak powder and Heico NH-5 which I also mix for paper- even if you use the same dilution for paper and film, mix separate containers for each. Choice of fixer for you may be more based on freight charges, and the easiest for that reason may be the Kodak powder.