High-speed Network Link between two Processors without Cable
The simplest solution in terms of design time is USB, where one SOM is configured as a USB HOST and the other as a USB DEVICE.
Also, if you have Linux on both the SOM, you may test this scenario using two PC's without writing a single line of C code.
When you design the PCB, take a special care when routing the USB traces because they are impedance-controlled ones.