Two dips can't share a seat

3 + 3 is spoken as 2 + 3

The third tone is a slow dip. Two slow dips in a row would take all afternoon, so Mandarin speeds the first one up into a rise. Nobody announces this; everybody does it.

Dictionaries and our word pages still write both as third tone. Your eyes see nǐ hǎo; your mouth says ní hǎo. Both are correct: one is the spelling, one is the sound.