Yes, you get the total wave amplitudes for both slits being open by adding the wave amplitudes you get for each slit being open, as long as there is nothing in the apparatus that can determine which slit the particle went through. This is just what you would do with sound waves or water waves also, by the way. (To clarify, wave amplitude is what each individual source of that wave produces, and you must add all the amplitudes from all the sources together to get the total amplitude, which then has both a magnitude and a phase. The magnitude is what the energy in the wave depends on, and the phase is what you think of as where the wave is locally in its oscillation. Since the contributing amplitudes at any given point can be 180 degrees out of phase with each other, they can cancel rather than add, and that is what produces interference fringes in the two-slit experiment.)