Oh Grasshopper,
Discussed this was in “tandem rigging” discussion in this section. But discuss if I may again.
Putting split shot on line when nymphing you are depends on where you want your flies to be underwater. If you put your split shot above your two flies, then your point fly (second or bottom fly) will ride higher through the water column and your dropper fly (first fly) will be deeper. If putting the split shot in between you do, then both flies will ride lower with the point fly trailing.
Weighting your tippet with bead head flies helps get them down without the hassles of split shot.
Experiment you must to know the secrets of tandem rigging. A lost ancient art form
