The Omega is DDR4
Maximus (which version) ?
DIMM.2 is not DDR-anything, it's a couple of NVMe M.2 storage slots that just looks physically similar to DDR. It is a proprietary ASUS thing, a way to mount a couple of extra M.2 drives perpendicular to the motherboard for both space and cooling.
On the original question, I don't know if the modules are compatible between different boards. On the one hand, there shouldn't be much need to change the pinouts on the slot between different boards. On the other, ASUS are constantly innovating and might have some reason that they did change the pinouts. I'd be cautious about using anything other than the DIMM.2 module that is included with the board. You also probably shouldn't use a DIMM.2 module from a PCI-E 3.0 motherboard on a PCI-E 4.0 system, as the module may not be capable of reliably operating at 4.0 speeds (if the pinouts are the same). The boards are likely only tested with the DIMM.2 module that ships with them, so using the "wrong" module probably puts you into untested territory where you could get some glitchy behaviour.
I am looking for an answer to the following question:
Is the size of the Dimm.2 module (and the slot) for ssd drives in Asus motherboards universal?
i.e. whether the module, e.g. from the Rog Maximus board, will be fit the Rog Omega x299 board?
Thanks in advance for your answers