You need to catch a nincada then have an empty space in your
party and at least one pokeball in your inventory. then evolve
nincada into ninjask and then in your empty space you have a
shedinja. so you have both.
Actually, Shedinja is not a Ninjask evolve, but it's still Nincada evolve. to get Shedinja, evolve Nincada to lv20 with an empty slot in your party! once it's evolve, Shedinja will be in that empty slot with just 1 HP. so, you will get both Ninjask and Shedinja in your party. hope this usefull...
It is the last evolution . This is the REAL infomation about how to get a sheninja . Get a nincada , then get AN (which is 1 ) empty slot in your pokemon belt and get a pokeball . No other ball . Only Pokeball . Which you can get at the mart with $200 . Then level up your nicada to lv20 . Then see it evolves . Don't get shocked when it evolves to Ninjask but not Shedninja . Check yr pokemon belt . Theres two of the evolution !! Ninjask and Nincada in your belt ! Congratulations !