Hello Daniel,
sorry for the delay - I did some tests to figure out how to solve your idea.
I was not succesful.
To control another Shelly with short/longpress by using URLs is no problem.
These URLs run perfect:
http://IP-of-shelly/relay/0?turn=toggle (short)
http://IP-of-shelly/relay/0?turn=on&timer=180 (long)
The same shelly (independent if the configuration is momentary or detached) shows no reaction.
It seems, the actual firmware does not support sending an URL-command to itself and receive/response corresponding.
Have you already tested with scenes?