Light some candles and play some soft music.
I don't think you can force them to pair up.
Clownfish change from genderless to male to female.
They can't reverse from female to male or from male to genderless.
FWIW, while this is generally regarded as true, I believe there have been some very sporadic cases where a known breeding female turned male. It's been a while, but IIRC, it was a Clarkii simplex clown, and it might have been Rod that recorded this.
Can someone with a better memory confirm?
Just let new one settle into tank life. Having both in breeder box may cause more harm than good.