First off, this is generally a bad idea. The moment that the Ethernet connection goes down (or the system providing the swap space shuts down/crashes/reboots), the system that is using the swap space will die because it can't access the swap space. You'll also see even more serious than usual performance issues when swapping (you would need 10GbE and really good NIC's to avoid this, at which point you're better off just getting more RAM).
That said, you've got a couple of options. The simplest and most reliable in my experience for stuff like this is ATAoE (ATA over Ethernet) or NBD. Unfortunately, Arch does not support ATAoE out of the box (it's a lot simpler to set up quickly than NBD, and only requires special software on the side providing the swap space), but NBD is supported. You would need to create an in-memory filesystem on the system with lots of RAM, create a swap file there, and then share that out with NBD and tell the other system to connect to it.
Using NFS to share the file would probably work also, but is likely to be more error prone and less efficient.