Standards for conducted EMI testing
Yes it is possible to join two coplanar 1m x 2m planes into one 2m x 2m plane for each axis using mu metal foil with conductive glue. (EMI tape) or several  machine screw fasteners with some overlap (wavelength for 30MHz is 10m )

The purpose of the 2x2m in each plane is to shunt any radiated fields that might interfere with conducted noise going into the cable to the dual coil LISN up to 30MHz.

Cables can act as receiving antenna to CM noise and may still become unbalanced with a LISN. So the ground plane provides some capacitance to shunt these high impedance fields. The LISN inductance raises this CM impedance with frequency while unintended radiating cable antenna efficiency increases with frequency.

It might look like this with the XY axes joined.