Model gets pulled apart when Rigify rig is moved

Question

I have been modeling a poly character. I made the wings separately from the model's body. Then I joined them using Ctrl+J as a union modifier making the wings disappear. The wings have their own armature, separate from the Rigify armature that is used on the body.

When entering Pose mode and moving the Rigify rig around the wings which were initially two separate parts that got joined together, get pulled apart into their original pieces even tho they have been joined into one object. (I couldn't use union as it would suck the wings into themselves).

The base model in Pose mode not moved:

Rigify hips moved forward:

Answer 1

Here is what is happening: Each mesh object is attached to its armature through an Armature modifier.
When you joined the two objects together, only the Modifiers of the active object were preserved, so the wings lose their connection to the second rig.

Generally speaking, using two separate armatures on a single character is asking for trouble. There may be a way to add the second Armature modifier by selecting the mesh, then shift+select the wing rig > Ctrl+P > With Empty Groups (this will keep the existing weights). Or you may have to add the second Armature modifier manually...

But even if you do this your rig may not behave intuitively. Try joining the two armatures together instead. As a rule of thumb use one rig and one armature modifier per character. The character itself can be split into many mesh objects.