Can I set up a layer triggered by holding *both* fn keys in Chrysalis?

On my old M01s I had things set up so I hold both fn keys to enable another layer. It worked by having the default layer as layer 0, then the regular function layer as layer 1, then the “double-function” layer as layer 2, then other layers like numpad after that. Each function key was set up, if I recall correctly, to increase the current layer by 1 while held, so holding either one gave me the regular function layer, and holding both made the double-function layer active.

Is this possible when setting things up in Chrysalis? I did notice a “layer shift while held” option under “layers and keymaps” which sounds interesting but doesn’t seem to be available. (I opened a ticket asking about that.)

If it is possible, how do I do it, and would doing so then stop me from using Chrysalis to make further layout tweaks later?

Otherwise, I suppose I need to prepare a custom firmware?

It seems the answer is no, and because of a combination of that and wanting to change the enabled LED plugins and a whole confusing chain of events I’ve ended up ditching Chrysalis and going back to doing everything in a firmware sketch.