How can FN key work in a modular keyboard with QMK layers?

[junaruga]

@Martynas

Then it occurred to me that I have a mapping of my own FN layer and I have some keys mapped to the part where you’d be having a keypad. But if it’s two separate keyboards then it wouldn’t be possible on the keyboard level. Also solutions like KMonad would also not work.

I don’t understand what you are saying, and what is your concern.

You meant that the two separated keyboards are problem to have another layer?
I have the external split keyboard called Moonlander running with the QMK. And the left and right keyboard PCB are connected with a cable from the both controllers. I haven’t openned the Moonlander. So, having the 2 controllers are my assumption.

And here is my current 34 keys layout. You see the “1 hold” key on the left bottom. The key changes the layer from layer 0 to layer 1 when holding the key. It’s like Fn key on teh current Framework Laptop. The key uses the feature of “MO (layer)”.

QMK Firmware - feature_layers

MO(layer) - momentarily activates layer. As soon as you let go of the key, the layer is deactivated.

And I am also assembling another external split keyboard called Sweep Bling LP now. The keyboard has 2 controllers (in my case, the RP2040 based controllers to run QMK on it). This picture on QMK repository shows the 2 controllers on the both PCBs.