We study microwave photoresistivity oscillations in a high mobility two-dimensional electron system subject to strong dc electric fields. We find that near the second subharmonic of the cyclotron resonance the frequency of the resistivity oscillations with dc electric field is twice the frequency of the oscillations at the cyclotron resonance, its harmonics, or in the absence of microwave radiation. This observation is discussed in terms of the microwave-induced sidebands in the density of states and the interplay between different scattering processes in the separated Landau level regime. Over the recent years many remarkable phenomena, other than conventional Shubnikov-de Haas oscillations [1], have been discovered in high Landau levels of two-dimensional electron systems (2DES). A lot of attention has been paid to microwave-induced resistance oscillations (MIRO) [2] and zero-resistance states [3] formed at the MIRO minima. Another notable effect is Hall field-induced resistance oscillations (HIRO) emerging under strong dc electric field [4,5]. Recently, it was demonstrated that dc field also induces strong resistance suppression [5,6] and zero-differential resistance states [7,8].Stepping from the inter-Landau level transitions due to microwave absorption and/or impurity scattering, MIRO and HIRO are controlled by simple parameters, ǫ ac ≡ ω/ω C and ǫ dc ≡ eE(2R C )/ ω C , respectively. Here, ω = 2πf is the microwave frequency, ω C = eB/m * is the cyclotron frequency, E is the Hall electric field, and 2R C is the cyclotron diameter. MIRO maxima + and minima − are found at ǫ ± ac ≃ n ∓ φ ac , φ ac ≤ 1/4 (n ∈ Z + ) [9, 10], while HIRO maxima (minima) occur near integer (half-integer) values of ǫ dc [14]. To explain MIRO, "displacement" [11] and "inelastic" [12] models have been proposed. Until recently it was believed that the "inelastic" mechanism, originating from the oscillatory correction to the electron distribution, dominates over the "displacement" mechanism, which is based on microwave-assisted scattering off of impurities. However, recent experiments [13] have shown that the "displacement" mechanism cannot be ignored.Remarkably, in addition to MIRO at integer ǫ ac , many experiments [9,15] reported similar features near certain fractional values of ǫ ac = 1/2, 3/2, 5/2, 1/3, 2/3. These oscillations were initially explained by a multiphoton "displacement" mechanism [16] which relies on a single electron simultaneously absorbing multiple photons. However, recently [17] it was argued that in the regime of separated Landau levels "inelastic" multiphoton mechanism dominates the response while at the crossover from separated to overlapped Landau levels, two single-photon "inelastic" mechanisms become important. The first of these mechanisms is based on a resonant series of consecutive single-photon transitions [18]. The other originates from microwave-induced sidebands in the density of states of the disorder-broadened Landau levels [17].When a 2DES is subject to microwaves and a strong dc electric field si...