\documentclass[crownsize,11pt,landscape]{memoir}

\trimFrame  
\settrimmedsize {15cm}{25cm}{*} 
\settrims{20mm}{34mm}

\usepackage{polyglossia}
\setdefaultlanguage{marathi}
\setotherlanguage{English}

\setmainfont[Script=Devanagari,Mapping=devanagarinumerals]{Jaini}
\newfontfamily{\Jn}[Script=Devanagari]{Jaini}
\newfontfamily{\Sb}[Script=Devanagari]{Shobhika}

\begin {document}

{\Sb As main font- } ज्ञानेश्वरी , वल्लभ

{\Sb As a specified font (this is how
  it shold look like)-}{ \Jn ज्ञानेश्वरी, वल्लभ} 
\end{document}