Hi Christine

You don't give your great-grandparents' name, but have you looked for them in the Warsaw address book?  The 1930 edition (which would have gone to press in 1929) is here: https://polona.pl/item/ksiazka-informacyjno-adresowa-cala-warszawa-1930,NTU5MzI5/3/#info:metadata. 

Best wishes
Peter Lobbenberg, London, UK
peterlob@...