The fusion in LF2v1.9 is based on ID-numbers: If the character with the ID: 7 (Firen) runs into the character with ID: 8 (Freeze) and they're both on the same team and have less than 33% of their hitpoints, they will fuse into the character with ID: 51 (Firzen), starting on frame 290. By changing characters' ID numbers, you can have different characters fuse!
The bad thing about ID-Fusion is that you always have to sacrifice the fusion of Firen and Freeze for the new one...