We have performed a spectral line survey called Exploring Molecular Complexity with ALMA (EMoCA) toward Sagittarius B2(N) between 84.1 and 114.4GHz with the Atacama Large Millimeter/submillimeter Array (ALMA) in its Cycles 0 and 1. Line intensities of the main isotopic species of ethyl cyanide and its singly ^13^C-substituted isotopomers observed toward the hot molecular core Sagittarius B2(N2) suggest that the doubly ^13^C-substituted isotopomers should also be detectable. We want to determine the spectroscopic parameters of all three doubly ^13^C-substituted isotopologues of ethyl cyanide to search for them in our ALMA data. We investigated the laboratory rotational spectra of the three species between 150GHz and 990GHz. We searched for emission lines produced by these species in the ALMA spectrum of Sagittarius B2(N2). We modeled their emission and the emission of the ^12^C and singly ^13^C-substituted isotopologues assuming local thermodynamic equilibrium.