Unfortunately, the X1 port is the only real PN port on the device. X2 supports a few profinet features like device detection (DCP) and maybe topology (LLDP), but it doesn't support IO. Not only do you need to use X1 for safety comms, X1 is used for all of your PN data. X2 still supports S7 connections, HMI/OPC/PG connections, TCP, etc
The brochures and marketing material is usually pretty vague about this difference, and the tech specs are super confusing for other reasons. X2 is called a Profinet port, whereas X1 is for Profinet IO. It's splitting hairs in marketing, but a huge difference in actual functionality.
They have PN communication cards for the 1500, but I don't think those support acting as an I-Device either. However, they can at least be a controller if the other PLC acts as the I-Device.
So yeah, I guess I should have mentioned that before: to use I-Device, the PLCs all have to be on the same subnet as the IO network, because it uses the IO mechanism. I hope this is rectified eventually. It seems silly to sell a PLC with 2 ethernet interfaces, but then only support IO on one of them. At first I assumed it was a step 1 sort of situation, and it would be fixed with a firmware update. 1500 is up to v1.8, and it hasn't been added yet.