Reactions of P‐chlorophosphaalkenes (RMe2Si)2C=PCl (1a: R = Me; 1b: R = Ph) with the disilane Me3SiSiCl3 (5) furnish diphosphenes (Cl3Si)(RMe2Si)2C–P=P–C(SiCl3)(SiMe2R)2 (4a: R = Me; 4b: R = Ph) by Me3SiCl elimination. The structure of the new compound 4b was confirmed by X‐ray diffraction; it displays crystallographic inversion symmetry. Monitoring the reactions with 31P‐ and 29Si‐NMR spectroscopy detected P‐(trichlorosilyl)phosphaalkenes (RMe2Si)2C=PSiCl3 (2a, R = Me; 2b, R = Ph) as the primary intermediates from reductive P‐silylation of 4a, 4b, and P‐[(trichlorosilyl)phosphanyl]phosphaalkenes (RMe2Si)2C=P–P(SiCl3)C(SiCl3)(SiMe2R)2 (3a: R = Me; 3b: R = Ph) as unsymmetric dimerisation products that rearrange to provide 4a, 4b in step III of the reaction sequence. This step (the P→C 1,3‐trichlorosilyl shift reaction) was mimicked by the synthesis of (Me3Si)2C=P–P(SiCl3)tBu (7), which rearranges into an unsymmetric diphosphene tBuP=PC(SiMe3)2SiCl3 (8). The bulkier P‐chlorophosphaalkene (iPrMe2Si)2C=PCl (1c) reacts with 5, eliminates Me3SiCl and thereby provides the first persistent acyclic per‐silylated phosphaalkene (iPrMe2Si)2C=PSiCl3 (2c) in an incomplete reaction. 2c exhibits an exceptionally large NMR coupling 1J(31P,29Si) = ±249 Hz. Within weeks, the mixtures of 1c and 2c undergo decomposition with loss of the P=C functions.