>>370268976
I'd always recommend Blender if you want to be more optimised and have less scuff. Its a bit harder than Unity but massively more optimised though. You'll want to learn Blender long term anyways
Here's a bit of a guide on how to do it but it assumes you already have some Blender knowledge so if it's unclear you could ask

Apply all transforms on the second armature if you've moved it around
Make sure the duplicate bones/vertex groups are named the same as the ones on the main armature
Delete duplicate bones on the second armature that already exist on the main armature
In object mode select both armatures and ctrl + j
Reparent any additional bones to their proper place
Either join the meshes or on the second mesh make sure the armature modifier is pointing to the main armature