Separate CMYK and PMS colors into separate layers

Hello, Henriette
First of all, I think that the technique “For a print project I need to seperate artwork into a CMYK layer and a PMS layer” is not so good.
Separation work is RIP work. I don't think designers should be involved in that.
Do not do this especially after the popularity of the Adobe PDF Print Engine (APPE), which supports transparent objects.

However, if you aspire to separate and divide the layers, you can try the old Postscript method
Roughly speaking, it is a method to export as a ps file of each color from "Print", create a high-resolution CMYK composite image and a spot color image in Photoshop, and link each to the Illustrator layer.
However, since PostScript does not support transparency, you need to make sure that it is correctly reproduced.

I do not recommend much the above method.
If you do it, you can try it.

If you think that this is troublesome, do your best and cut out the spot color part with a pathfinder etc. and divide it into layers.

Why? It can totally change the look of the artwork.

Suppose you have a single layer with 3 objects, 2 spot objects that go over and under a 3rd CMYK object.

Moving the spot objects to a layer above or below the CMYK object will change the design.

I'd suggest making 2 layers, put all of your cmyk objects on one layer and lock it, then select all of your PMS objects and move to the other layer.