I'll go ahead and guess Zorn and Thorn, even though it's wrong.
The gaiety seems lopsided
- Zorn and Thorn are clowns. All happy and they dance around. But there's always something wrong. Or this could refer to the celebration when you defeat them.
As the two taciturnly join
- They get their asses handed to them. Then they become Multigemini whilst unconscious, or silent. Eh? Eh?
An act of necessity, yet not so
- Well, they just got beaten when separate. So they join because they have to in order to win. "yet not so" is a bit

for me, so I'll just say that they didn't
have to combine, because Zidane wasn't gonna get his bollock knife out and kill them when they're out.