Hi,
Please check if you can see video preview of the tow sources:
$ gst-launch-1.0 nvarguscamerasrc sensor-id=0 num-buffers=300 ! 'video/x-raw(memory:NVMM),width=1920,height=1080,framerate=60/1' ! nvoverlaysink
$ gst-launch-1.0 nvarguscamerasrc sensor-id=1 num-buffers=300 ! 'video/x-raw(memory:NVMM),width=1920,height=1080,framerate=60/1' ! nvoverlaysink
If the sources are good, please try
$ gst-launch-1.0 nvcompositor name=nvcomp sink_0::xpos=0 sink_0::ypos=0 sink_1::xpos=1920 sink_1::ypos=0 ! nvvidconv ! nvv4l2h264enc ! h264parse ! matroskamux ! filesink location=stereo.mkv nvarguscamerasrc sensor-id=0 num-buffers=300 ! 'video/x-raw(memory:NVMM),width=1920,height=1080,framerate=60/1' ! nvvidconv ! nvcomp.sink_0 nvarguscamerasrc sensor-id=1 num-buffers=300 ! 'video/x-raw(memory:NVMM),width=1920,height=1080,framerate=60/1' ! nvvidconv ! nvcomp.sink_1