Not sure why this is so hard to diagnose. Because the OP won't share where it's leaking. let's start with eliminating the components. also, anyone suggesting teflon tape does not know how water hose connection work. The threads only pull the sealing surfaces together against the rubber gasket.
If you attach a garden hose to your bib with a device at the end that shuts off, does it leak? Where??
If out of the bib stem, the packing seal is bad.
if out of the rubber seal, you have a bad rubber seal or it is not tight enough. Use quality rubber gaskets and or use channel locks to really tighten. If hose has plastic nut, they are junk!
If the hose attempt above is now leak free, Now try your splitter. With both side of your splitter closed, dose it leak? If yes where? Around gasket or via the ball seal?
If gasket, same as above tighten with plyers/ plastic nut + junk.
if ball seal, junk