In the model viewport set the view to camera.
Split the viewport.
Dupe the current camera in the item list.
Nest the duplicated camera as a child of the original.
Set the desired numerical offset for duped camera.
Control the pair via the parent camera and they move in perfect synch.
If this all goes according to plan, see if you can model in one viewport and see it simultaneously occur in the other. The offset I want for the duped camera is for creating a stereo pair, or a virtual 7 cm apart which is the distance between two eyes. I have no idea about how to do this camera offset in that exact 7 cm distance amount, but try it anyway.