combine(copy,transfer) weight from the armature to the another.

the old mesh and weight in old armature transfer to the new armature.

maybe using for wearing dress.

according to

using blander 2.79

if you need the same location, maybe you can use the different layer.

Three bone conditions from the old armature to new armature:
i.the new bone
ii.the same bone(it means the SAME NEME and function) using bone

1.take apart the mesh from old armature. (alt+P in object mode)

2.take apart “i.the new bones” from the old armature. (P in edit mode)
(you will find “i.the new bones” appear in the new armature.)

3.the separated bones(armature) join to the armature you want to transfer to.
(pick the new armature(the separated bones) in object mode, pick the armature you want to transfer to in object mode, ctrl+J)
focus on the order! you can check the result from the outline windows.

4.let the separated mesh be set parent to the armature, by “With Empty Groups” way!!! “With Empty Groups”!
(both in object mode, ctrl+P)

5.adjust the added bone to the proper parent.

1.the same bones from two armatures you want to transfer(copy) weight MUST be the same name.
2.when you join the bone, if they have same name, the added bones’ name will add “.001” or “.002”…etc in the bone name in the tail.
and you can use outline windows to find the bones with “.001” “.002” (maybe searching or expanding all by pressing “num+”; the"num-" is collapsing),
and then let the ".001"etc bone’s child bone be set parent to proper bone.
finally delete the ".001"etc bones.