SG like other brands that do dual cameras, well they can only operate with the specific rear cameras sold by those brands, and rear cameras that work with that specific model.
The SGGCX2 pro i believe can operate with 2 of the SG rear camera offerings, the regular one or the waterproof one ( SGR291CAM or X2WRC323 )
SGR291CAM - Optional Rear Camera for SGGCX2PRO/SG9663TD - Also Replacement Camera for SG9663DC/DCPRO/DCPRO+
streetguardian.info
X2WRC323 - Optional Waterproof Rear Camera for SGGCX2PRO
streetguardian.info
So i can not take a camera from my K2S system and pair that with the SG9553DC camera of mine, only optional rear cameras from SG work with my SG9663DC front camera, and my old 9663DC i believe only have 1 option for the rear camera,and that's the one that come with this dual system.
It is only in some cases like CCTV cameras you can bundle different brand cameras together into a system, but all cameras have to support the standard the system with with, in CCTV cameras this could be one of the ONVIF standards or one of the many other CCTV standards.
Some gopro you can also link together, but not with a SJ camera or DJI, it will have to be the same gopro cameras.