Millimeter/submillimeter spectra of PH_2_CN(~{X} ^1^A') and CH_3_PH_2_(~{X} ^1^A') have been recorded for the first time using direct absorption techniques. This work extends previous measurements of both molecules beyond the 10-50 GHz range. Both species were created in the presence of an AC discharge by the reaction of phosphorus vapor and either cyanogen and hydrogen (PH_2_CN) or methane (CH_3_PH_2_). Twelve rotational transitions of PH_2_CN were recorded over the region 305-422 GHz for asymmetry components K_a_=0 through 8. For CH_3_PH_2_, eight rotational transitions were measured from 210-470 GHz with K_a_=0 through 16; these spectra exhibited greater complexity due to the presence of internal rotation, which splits the K_a_=1, 2, and 3 asymmetry components into A and E states. Combined analyses of the millimeter/submillimeter and previous microwave data were performed for both molecules. For PH_2_CN, the spectra were fit with a Watson S-reduced asymmetric top Hamiltonian, resulting in more accurate rotational and centrifugal distortion constants. In the case of CH_3_PH_2_, an asymmetric top internal-rotation Hamiltonian was employed in the analysis, significantly improving the rotational and torsional parameters over previous microwave estimates. Searches for both molecules were subsequently conducted toward Sgr B2(N), using the 12 m telescope of the Arizona Radio Observatory (ARO). Neither species was identified, with abundance upper limits, relative to H_2_, of f(PH_2_CN/H_2_)<7.0x10^-12^ and f(CH_3_PH_2_/H_2_)2 and >200, respectively.