Ammeter and controlled sources in different subcircuits in LTspice
It may be possible, with some abuse of notation, but should you really do it? 

Like software functions, the whole point of subcircuits is to isolate their internals from things going on outside.

The correct way to do this would be to add the controlling and controlled signals to the pins of the subcircuits, and wire them up in the schematic.

This obligatory XKCD came to mind



The hover text for it is


   All I want is a secure system where it's easy to do anything I want. Is that so much to ask?