The answer is yes, because you are adding capsaicin with each pepper.
How do you think they make pepper spray from cayennes? It's extracted and combined and when packed into a spray bottle it is 1,000,000 SHU, not 30,000, the average SHU of a cayenne.
Scoville units are a measure for capsaicin content per unit of dry mass. The more capsaicin per unit, the hotter the pepper. If you pack the heat of two jalapnos into one and have it tested, it would show that you had a "double heat jalapeno" so eating two at the same time vs. one will result in double the sensation. The more peppers the hotter, makes sense, right?
Let's say a jalapeno is 2,500. You dice it up and put in on your pizza. Now dice another 2,500 rated pepper. The pizza doesn't magically know it's a different pepper, but by Scovie's logic you just added 2,500. But you added 5,000. However this "adding scovilles" is not really correct, it's a measurement of heat, and an average per pepper.
If you want a 1,000,000 SHU pizza with jalapenos you'd have to add 400 jalapenos. And you'd have to fit it all in your mouth at once to equal the sensation of one slice with a naga.