Maxwell Stress Tensor at material boundaries

I'm not sure discontinuity has anything to do with it.

If you have two parallel waveguides running in the x direction separated by a distance d with the electricomagnetic field having momentum inside the guides pointing entirely in the x direction then for every little piece of matter inside the waveguide the force density for components other than the x component is given by the divergence of the stress tensor (other than the x component of the divergence).

Thus the total force on waveguide 1 in directions other than x is the surface integral $$int_{in1} n_jT^j_{i,in1} dS,$$ for $ineq x.$

In the x direction the stress tensor tells us only how the field and mechanical momentum have their sum change, and so there isn't as simple a relationship between stress and mechanical momentum (since there is field momentum in the x direction which is possibly changing).

Now we basically found the force on the guide by looking at just the force on the body (inside) of the guide. That is not always OK, so let's check if that was OK.

Well, if there is surface charge or surface current then we need to consider not just force density but also force per area (and if we have line charge then we need even more) but if our waveguide has no surface current or surface charge then we are done. This is because the mechanical momentum changes because of the Lorentz Force and hence we need the charge and current if these are volume charges and volume currents we get all the charge and current by considering little volumes and adding them up.