It is an interesting question and I did not know the answer either so I decided to try it. 
I tried to send the LFO of Zone 1 to Bus 1 and then, in Zone 2, I assigned Bus 1 to the Level.
It did not work… 
It appears that the modulation signals are only available within the zone. But, what parameter are you trying to use as a source that is NOT available in the other zone? For example, if you have an envelope or LFO in Zone 1 that you want to use in Zone 1 AND Zone 2, you can just duplicate the LFO or envelope in Both Zones (Or just select the layer and create the LFO/envelope there and it will duplicate the settings in both zones for you).
I am just trying to establish a use case to see if there is a way to do it.
J.L.