There’s a lot of things going with sound, but in general sound1 + sound2 is louder. You may not always perceive that, and if one of them is significantly louder, it’s not a significant difference. Active noise cancelling uses sound to make other sounds disappear, but this is just a trick, the pressure is still there (it’s why prolonged use of active noise cancelling can be bad for you).

When it comes to decibels, dB, it’s a logarithmic unit so adding them isn’t straightforward. You need to do logarithmic addition, so 5dB + 5dB = 8dB, and 90dB + 90dB = 93dB, but 70dB + 90dB = 90dB.