"I can explain how to set up a jcm in another post - I ran into some issues with it for M4 as it doesn't behave the same as with other Poser figures for some reason and I was trying to figure out why."

Don't use Create New Master for the crosstalk empty master in M4's body, spawn an empty morph in the body to use as the crosstalk.

So, for instance, the JCMs in the shirt are named rShldrBend80JCM/lShldrBend-80JCM let's say (internal names)...

Do this, with the M4 figure completely 0 go to figure menu Spawn Full Body Morph. Name it anything unique, we'll change this in a bit.

Your Blank FBM will appear, every actor in M4 will now have a local empty (including parented props)... right click the FBM dial and Delete Morph. Select all and then deselect just the empty in the body. Delete. You're left with just an empty master in the Body.

This is a targetGeom rather than a valueParm generated by the Create New Master command.

Do some hidden cleaning up, in the Hierarchy select the goal center of mass props and delete those empties that are left hiding in there... remember I said it generates an empty across all, including parented props. The initial delete morph action doesn't include those empties in the props. This goes for controlHandleProps also.

Go to that empty targetGeom you have and activate the dials settings. Rename as desired insuring that the internal name matches the internal name of the target JCM in the shirt, so in this instance it would be rShldrBend80JCM. You can rename internal names by clicking the internal name field. Set the limits min 0/max 1. Note that when you rename internally when you close the settings it will plop that dial down to the morph group, so if you had it in any other dial group you'll have to move it back there.

Ok, so now we have this empty targetGeom master sitting in the M4 body, named internally to match the internal name of the target JCM in the shirt.

Go to the zRotate dial of M4 rShoulder (Up-Down). Edit dependencies. Start teaching. Set the shoulder z rotation to 80, set the targetGeom dial to 1. Set the z rotation to 0, set the targetGeom dial to 0. Stop teaching. You now have an empty targetGeom dial in M4 that sets to 1 when the shoulder is set to 80 degrees. Being that this is internally named to match a morph in the shirt, when M4's shoulder is bent to 80 degrees the named dial will be set to 1 (thats all it does, sets a dial value to 1, no other action in M4) consequently the liked named morph in the shirt will be set to value 1. That is if you have include morphs when conforming checked in the conformer's properties.

Repeat for the left target JCM.

This should solve the problem.