You can get both Shellos East Sea and West Sea, but they are each exclusive to either Omega Ruby or Alpha Sapphire
Shellos West Sea is exclusive to Omega Ruby.
>Shellos is available in both, but West Sea Shellos is exclusive ~ Serebii
Shellos East Sea is exclusive to Alpha Sapphire.
>Shellos is available in both, but East Sea Shellos is exclusive ~ Serebii
So you, hiving ΩRuby, will be unable to get the blue Shellos (East Sea) without trading.