You cannot really do this with Keyboard Maestro.
See the wiki article: Can I use a second keyboard solely to trigger macros?
Keyboard Maestro has two primary key triggers: hot keys and device keys.
Hot keys intercept and swallow the key press but cannot distinguish between the same key on different keyboards.
Device Keys can distinguish the key on different keyboards, but do not swallow the key so the key continues to perform its normal function.
So you could use all the function keys (with associated modifiers) on both Macs using a device key trigger. But you could not use, for example, the “a” key on a second keyboard without also typing an “a” (or swallowing both a’s if you used a hot key).
An alternative is to use a non-keyboard keyboard, something like an XK-24 or the like, that does not perform any normal function. You can use device key triggers with that kind of keyboard.