待ち合わせ　is a combination of two words: 待つ　(to wait) and 合わせる (to bring together). So if you look at the literal meaning, it's like "waiting for each other."] That's why 待ち合わせ is used when you set an appointment to meet with somebody. The verb is 待ち合わせる by the way. On the other hand, 出会い is a combination of 出る (to go out) and 会う (to meet). Again, literally the person goes out and meets somebody. But there is no connotation of an appointment unlike 待ち合わせる. If you meet somebody else while you're out by coincidence, it's like fate. This is why 出会い is ONLY used when meeting someone for the first time. You can't use it for someone you've already met, unless you were talking about the first time you met that person. Also, 出会い is a noun. The verb is 出会える。