In modern electronic warfare, array radar systems apply adaptive beamforming techniques to form nulls in the beams toward the jamming angles for multiple jamming cancellation. However, the nulls lead to distortion, especially within the mainlobe, which severely degrades the target detection ability. To address the degradation, this letter proposes joint adaptive beamforming techniques for distributed array radars by two-stage adaptive processing. First, the linearly constrained minimum variance beamforming is performed for multiple sidelobe jamming cancellation with mainlobe maintenance within each array radar. After that, joint beamspace processing with multiple radars is carried out for multiple mainlobe jamming cancellation using adaptive-adaptive algorithm. Excellent performance in both jamming cancellation and target detection is attained at very low computation and data transmission cost between multiple radar sites. Detailed evaluation and simulation results are provided to validate the proposed technique.