well if you think about it, a real mech of such sort will have some “joint” around the waist that can freely rotates in certain axis. if you use 2 skeletal meshes, the transform of the upper body’s s.mesh is effectively this joint (assume the legs are the root). If you use just 1, then you just need to locate this joint in your model and manipulate freely. both ways can implement it in a similar way I believe.
The more complex part in my opinion would be blending the animatinos so that they natural which will depend a lot on how you design your model and animations