You don't just need the cameras to see each other, you need them to be perfectly parallel to each other as well, as otherwise they're photographing along a different plane, which may give conflicting results.
Suppose camera A has line L_A on the camera. A’s optics and marks are both on L_A, so A’s image plane contains L_A and camera B’s image plane also contains L_A (you know the latter because you’ve aligned the cameras so camera B sees L_A). And vice versa: camera A sees L_B. In 3D Euclidean space, two distinct lines define a plane, and both camera’s are photographing planes that contain L_A and L_B, so both cameras are photographing the same plane.
More concretely, if the cameras are photographing along different but parallel planes, then they won’t see each other.
Your solution works in a geometric world, where light propagates in a perfect straight line of infinitesimally small width. That's not true in reality, where light propagates in an ever-expanding cone.
More concretely, you can have two cameras photographing along different but parallel planes that do see each other.
