Automator / Apple script (to trigger a macro on a second Mac using Keyboard Maestro)

Your request is very specific, using some uncommon applications, so it's unlikely you'll find a direct solution.

However, AppleScript has a mechanism to send AppleEvents to an application on a remote Mac. It's gotten a little trickier over the years as security features have blocked common paths, but it's still possible to do.

You'd have to first work out the commands you want to execute on the remote Mac (assuming that Stream Deck is scriptable), Then configure the remote Mac to accept Remote Apple Events (System Preferences -> Sharing), then setup the local Mac to send the appropriate commands over the network..

Tricky to prototype and debug without the actual setup, which is why you'll largely be on your own, but it may be doable.

Automator is not a distributed tool and lacks specific actions for communicating with other networked Macs. You will likely need to establish peer Mac security to enable an Automator Run Shell Script action to SSH into the other Mac, and run an executable or shell script on the other Mac. Your request is out of scope for what we can provide here.