I think previously it would have been sublet/share, but this may well be changing in areas where these arrangements are more common. Your understanding seems correct to me, as long as piso can mean apartment.
No, you're right, and it's a very awkward phrase to me too. I didn't explain myself very well. Also, I don't live in Brooklyn.
Sublet is often used as a noun -- so a sublet can be shared. Is there a noun for "sublet" in Spanish?
I only meant that I think 'a sublet' can refer to an apartment, or a flat, a townhouse - so, I didn't want to tell you that your translation is correct if 'piso' specifically translates as 'floor' or 'level' of a building. But overall, your understanding seems right -- un apartamento o piso subarrendado y compartido.