Parameter mirroring.

[fhill2]

How can I set up a cross-track macro? I'd like to map both releases from my 2 synths on different tracks to 1 knob and control it from there. (I know I could host 2 synths in a rack and map them to the same macro) but by using 2 tracks I can use send return fx.
Maybe this is possible by mapping both parameters to a controller? or observing API changing and forwarding them onto the other parameter?
I know how to do this in M4L, but I have no experience in Bigwig scripting API.