I do not recommend to prepare separate A, B solutions of XTOL, Microphen, ID-11 and probably many other packaged powder developer. The part A usually contains reductants and possibly acid, but little or no preservative. Even in mildly acidic (boric acid etc) solution, metol and phenidone do get oxdized and the stock die quickly, and I won't be surprised if it completely dies in a few weeks.(posted 8226 days ago)Sulfite is usually packaged with alkali in part B. Why do they do that? Because if they added sulfite in A, metol won't dissolve for a very very long time.
If you can make a double concentration, it might be a good idea. But I recommend you to test keeping a small sample in a refrigerator for a few days to see if any crystal forms. If you get cristallization in the concentrate solution, they are very difficult to dissolve again even after dilution (that's partly why we don't prefer crystalline form when mixing chemicals).
I think the best suggestion I can give is to mix the stock solution as instructed, and store then in many small bottles completelly filled up. Make sure the bottle caps are very tight. With PET bottles it's easy to check. Most bottled water use very good bottle and cap, and you can test them by squeezing very hard.
Ultimate solution in minimizing wasted chemical is to mix from scratch.