Blender - Connecting vertices of 2 different meshes

[SK5]

As the title says, i need a bit of help. I recently got into modeling and im facing my first MAJOR problem today.

 

I started off with a little bit of frankensteining. Problem is that after replacing the arms of my model, the placement is a little bit off. I have tried using a "MultiEdit" addon but that messes everything up, tried using ctr+j to merge the meshes and then split but that screws everything up. 

 

So my question is, how do you connect vertices of 2 different meshes without screwing up parenting/other stuff?

[Boothand]

Two different objects can't share vertices. If you need them to be at the exact same location, you can use vertex snapping.

For correct deformation, they need the same bone weights.