man my blender knowledge has deteriorated over time, having a simple problem that I know should be a quick fix but I'm drawing a blank.

I have a mesh that has an armature and they work together fine in pose mode, the bones moves the mesh. I then import a second mesh with an armature and it also is working fine. I fix model with cats just so everything is good, everything still working fine. Now what happens is when I join the two armatures together with ctrl+j, the second armature like gets "unbound" from it's mesh. Meaning I can move the armature in pose mode, but the mesh it was connected to just stands there. They work fine before connecting but once they all connect it borks. Halp