It sounds like you have only one part of the roll exposed differently. I guess you could load it on the reel such that the part exposed at 100 asa will be on the outer most portion of the reel. (ie the part that will show density earliest in the development)(posted 8062 days ago)Inspect (briefly) under a green safelight when the time for 100 asa is up, cut the roll after you see the last frame with a faint immage and continue with the development for the 400 portion. Immediately stop the part exposed at 100, then procede to the fixer. Of course you run the risk of fogging the roll.