your left image is invalid / empty. please do NOT throw away the return values from cap.read() , but check them !
sorry, but this is a bit unclear. where is the problem, exactly ? can it be, you get both streams concatenated horizontally from a single capture, like the image you show above ? in that case, you have to split it into 2, not use 2 VideoCaptures
Resolution & frame
MJPEG：2560X960@ 60fps/2560X720@60fps /
1280X480@60fps /640X240@60fps
^^ this looks pretty much like it !