There are 2 ways to fuse. Any other way you hear "is a load of bull crappy" (as my science teacher says).
Here are the ways...
The first way is by using the Porta Earings. ALL the Kaioshins wear them. Once you put them on, you are fuse FOREVER, unless something "weird" or unexpected happens. I don't know if the Kaioshins are fused, but they do wear them. Vegetto is the result of Goku and Vegeta using these earings.
The other way is to do they fusion dance. To do this, the 2 people must have the same powrlevel, and know the dance PERFECTLY or they will turn out REALLY weak. Gotenks is made from Goen and Trunks fusing with this dance. Also Goku and Vegeta do this to make Gogetta.
- Characters who fuse:
- Goku + vegeta = Gogeta
vegeta + Goku = Vegetto
Goten + Trunks = Gotenks