This video shows how you can retarget biped/human animation and motion capture onto characters that have extra sets of limbs in Unreal Engine 5.7. Using standard, Mixamo source animations, the video shows how to apply different arm mocap animations onto a mech robot that has two sets of arms, so that each set of arms animates independently from different source animations.
https://dev.epicgames.com/community/learning/tutorials/DElK/retargeting-animations-to-multiple-sets-of-limbs-in-unreal-engine-5-7