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$\begingroup$@Uriya, I saw this in the hypotheses of the covering morphism $pr$ in I.1.3 of the book 'Spectral decomposition and Eisenstein series' by Moeglin and Waldspurger. I guess that the authors made this hypotheses keeping the double covering map from metaplectic group to symplectic group in their mind.$\endgroup$
$\begingroup$Yes. This is true in general. In fact, the definition of automorphic forms on Mp depends on this splitting. If my memory is correct, Moeglin and Waldspurger dealt with more general case than the metaplectic double cover Mp.$\endgroup$
$\begingroup$@Qing, Thank you very much! I referee the book ‘Spectral decomposition and Eisenstein series' by Moeglin and Waldspurger but couldn’t find proof of the generalization of this. Would you please let me know the page of the book?$\endgroup$
$\begingroup$I don't think Moegline-Waldspurger has a proof. The proof is probably in one of the two Weil's classical papers. The point is, MW has to assume there is such a splitting because they considered more general covers. For those more general covers, the existence of such splittings is not guaranteed. Since you only ask the metaplectic cover, the answer should be yes.$\endgroup$