Duskblade and stormrazor dont stack

Went into training tool with nocturne and master yi. Leveled both up to 18 and bought duskblade of drakkthar and stormrazor. put down a dummy and autod 1 time to see damage of both. went to store sold duskblade and bought youmuu's ghostblade. went back to dummy and auto'd 1 time to see exact same damage as with duskblade despite duskblade proc being active.