You simply make them use the same skeleton asset. They need to have the same general structure and same bone names, but proportions don’t need to match. Read this for all the info: Skeleton Assets | Unreal Engine Documentation. This will allow them to share the same animation assets. You can set the skeleton either on import or in the skeletal mesh after import.
When the proportions are different, you will need retargeting. However, since the meshes both use the same skeleton, the retargeting will be applied to the skeleton asset and not the animations. Read these on how to do that: Animation Retargeting, Using Retargeted Animations, and this video is what I followed: Skeleton Assets: Importing, Sharing Skeletons & Anims | 02 | v4.8 Tutorial Series. It’s simple & fast and only needs to be done once on the skeleton asset.
Any other touchups you do in the animation blueprint.