Set bonuses in ESO do stack if they are not named bonuses. For example if you get Major Sorcerey (20% Spell Damage buff) from two different sources, only one counts. However, if I get Major Sorcery and Minor Sorcery, they will stack because a Major + a Minor can be stacked together. You can also combine Major Sorcery with an unique spell damage buff, from example from the set Law of Julianos, which has extra spell damage listed on the 5 piece bonus without it being a Major or Minor buff.