Hmm from me too! Just expanding on some points in case not clear. A trigger can be connected to multiple layers, or you can have multiple triggers triggered by the same key ('z' in your case). I could well believe when you have one trigger with multiple layers in it, you will get one control button. When you have multiple triggers that all independently listen for the 'z' key then there will probably be multiple buttons in the control panel.
To add a second layer to an existing trigger, just drag the layer (in the rigging panel) from the hierarchy to the trigger. A trigger can have many layers at once (some mistakenly think a trigger can only control one layer at a time). To remove, there is a little 'x' next to the layer name to remove it from a trigger.
The funky part is when you get into swap sets. Normally to keep things easy to understand I put all the alternatives for a swap set under the one group. But if it is left and right hand you cannot do this - they need to be under the left and right arms.
I am not at a computer just now, but what I would try is to create triggers for the left hand with whatever swapset for different hand positions you want. Make sure that works. Next, on the right hand, drag the layers over to the left hand triggers you already created. Do not create a new swap set. Hopefully the left hand swap set will manage both hands and the same time.
i hope I understood what you are trying to do correctly...