How to join 2 armatures correctly?

I have two armatures in the form of two separate objects. When I connect them (ctrl + J), for some reason, for one of my armatures, the bones rotate at some (small) angle. I have already applied both the pose and the location with the scale, anyway this happens.

You didn’t mention applying the rotation. Is there any local rotation on either armature?

Another issue that can create problems is if the armatures are parented to any other object (which can have rotation, scale, etc, inherited by the child.)

Hey. Yes, I have some (or maybe all) bones (I repeat - bones, as parts of reinforcement - and not all reinforcement, like a set of bones) - rotate 10-40 degrees.

I’m afraid I don’t understand what you’re saying. If you link a file, I’ll take a look at it.

Your armature ‘object’ should both read 0,0,0 on location, rotation and scales before joining them.