Stacking or "Piggybacking" of headed shear studs ACI 318-19 R22.6.6.2

I don't have the code on hand right now, but the size effect factor can be taken as 1.0 for two-way shear if you provide some quantity of reinforcement (studs or stirrups). It's below one of the punching shear capacity tables in Chapter 22, they specify how much of either type of reinforcement, so my guess is that if you have deeper slabs that need taller studs than are manufactured, you can stack them as the commentary indicates to still take advantage of the size effect factor being 1.0.

One issue that came up for me recently was regarding the outer punching shear perimeter past the reinforcement still needs to use the size effect factor, which forces you to use larger punching shear perimeters than in previous codes for thicker slabs.

I can find that code reference for you tomorrow, but it's somewhere near the end of the punching shear section.

Agree with Luceid. Manufacturer's only produce certain heights of studs so "piggy backing" is used for deeper concrete sections.

Here is something that is relevant.