If you put both assemblies into the same top level assembly and make them both flexible you might be able to create a mechanical linkage between the joystick and robot arm to get it to move. You can hide this mechanism after you get it working.
Knowing how joysticks work and ow arms move I really doubt that this approach will even give you a motion you would be satisfied with. SWX is a mechanical system and you are looking for an electrical to mechanical analogue. There are some very sophisticated things I have seen done with external motion controllers moving a mechanism in SWX. It would be physically connecting a joystick to your computer to move the arm. I guess if you can do that (I saw the equivalent of that at SWW2013 in Orlando so I know it can be done) you could have the signals from the physical joystick also move the SWX model of the joystick. If then one outside joystick can make two different SWX assemblies move at the same time you would think there would be a way to achieve just what you are asking.
I am not that guru and I haven't seen very much in the way of that type of discussion on this forum. Perhaps using the Motion Study and sensors you can achieve what you are after.
Your request is reasonable and intriguing. Please keep us posted of what you eventually do to solve this.