The easiest way, I think, would be to duplicate your path splne, move it over in X, align a second camera to it, and then drive the offset along the spline with the other camera’s align-to-spline offest. To do that, right-click on the offset parameter and select Animation>Set Driver from the menu. The go to the other camera’s offset parameter and select Animation>Set Driven (either absolute or relative).
That said, I think stereographic production is a bit more complicated than using two cameras. I haven’t done much of it, but it can be fairly involved. It’s built into R13+, but there are some free stereographic rigs available for earlier versions, e.g., https://www.svoigt.net/index.php/downloads/23-downloads/57-free-stereoscopic-camera-rig-for-cinema-4d