Kerr frequency comb generation from a nonlinear high-Q resonator becomes an interdisciplinary research topic emerging from nonlinear optics, integrated photonics, and ultrafast optics. We show that ultrashort cavity solitons can be generated from a mode-locked Kerr frequency comb in a dispersion-engineered nonlinear microresonator. The spectral flatness of the comb is greatly improved by making the cavity soliton as short as two optical cycles, with a comb line power variation below 20 dB over an octave-spanning bandwidth from near infrared to mid infrared, while excellent spectral coherence is achieved by soliton-based mode locking. It is shown by simulation that the two-cycle solitons are robust to the wideband soliton perturbation effects such as all-order dispersion, frequency-dependent Q-factor, dispersive wave generation, Kerr self-steepening, and stimulated Raman scattering. The pump power used to generate an octave-spanning combs can be significantly reduced when a dispersion profile with four zero-dispersion frequencies, which paves the way to achieve a fully integrated frequency comb generator on a chip.