Why can I only add a D435 to my shopping basket and not also a D415?
You could try doing the pre-orders separately, registering the other one once the basket has been emptied. I'm not aware of a limit on pre-orders, but if there is such a term then I would first register the model that is most important to your project.
I heard that works, but that will also double the shipping costs.
Another thing you could try is putting one in the basket and then closing the browser, opening a new browser window and going to the store again. If you do this, hopefully the first order will still be in the basket as your login has not expired, and you can then try adding the D415 to the same order.