The guardian of the Earth split into 2 forms: Kami (pure good) and Daimoa (pure evil). Daimoa had a son, Piccolo, who fused permanently back with Kami. This was what he meant when you fused- that he was neither Kami nor Piccolo, "just a Namek who had long forgotten his name".

Daimoa and Kami were originally one. If one died, both died. Daimoa had given birth to Piccolo at the time he was about to die, so in essence, Piccolo was Daimoa. This is why Kami didn't die when Daimoa got killed. Piccolo made up for the link.

Piccolo permanently fused with Nail.

The fusion technique used by Goten and Trunks to form Gotenks is temporary and can only last for 1/2 hour.

The fusion technique used by Gokou and Vegeta to form Vegetto is permanent, but the effects were cancelled out when he was eaten by Boo.

Fat Boo separated into two Boos. The evil Boo was killed and reincarnated into Ubuu. When good Boo was killed by Vegeta-Bebi, his particles fused with Ubuu.

When Goten fused with Trunks to form Gotenks, Gotenks ended up wearing the fusion suit, as did Ubuu when he fused with Boo.(9)