First fusion-
Trunks and goten (majin buu saga)
trunks and goten fusedby using the ear rings that supreme ki gave them to try and beat majin buu there fusion was not success full at first and went fat, but after that the fused properly but there power was still not anuff to over come the great power of majin buu and were beaten.

Second Fusion-
Goku and Vegeta (majin buu saga)
Both Goku and Vegeta fused by using the ear rings that supreme ki gave them to beat majin buu and formed Gogeta to try and beat majin buu, they were doing very well and were kicking his ass then as time went on they ended up loseing and lost again.

Third Fusion-
Goten and Trunks (Movie 12- The Rebirth of Fusion!)
Goten and Trunks fused during movie 12 when Goku and Vegeta fused.
They turned in Gotenks who defeated the dead who were reawoken from
hell. Gotenks, later was able to turn into the Super Saiyan 3 form,
which only him and Goku reached.

Fourth 2-
Goku and Vegeta (Movie 12- The Rebirth of Fusion!)
Goku need to fuse with vegeta in order to beat the evil creature
that was formed after fat Jeninba was killed by SSJ3 Goku. The first
time they fused they messed up because Vegeta didn't have his fingers
pointed up while doing the fusion dance. They turned into fat Gogeta
who farts and is weak and slow. They split up again and joined
correctly into Gogeta and killed the red demon and send the dead back
to their graves.

Fifth Fusion-
Goku and Vegeta (last dragonball ever)
Even at ssj4 both vegeta and goku with ther assume strength could not beat the first dragon, so the used fusion at ssj4 and formed the ultimate Gogeta (looks bloody assume) and smashed the shit out of the first dragon, then as they were about to distroy the first dragon the fusion were off as they were fulling around to much and fusion time was up.