Trust that you are doing good!
As a workaround, you can make use of Mapping Load and ApplyMap functionality to get the "f" from second LOAD if first LOAD & second LOAD is related to each other (To do so change the LOAD order i.e. first LOAD becomes second and vice-versa). Post that, in first LOAD create a composite Key (a & '|' & f AS Key) to utilize it in further joins.
Hope this will be helpful.