They’re probably just broody. It’s important you break them soon especially if they’ve been broody that long. It’s not good for their health. To break them you would need to put them in a wire bottomed cage for a few days or week. The air flow underneath prevents them from being able to keep their imaginary eggs warm.
As for them being in the same box, it’s not that their in love or anything like that. Chickens just naturally like to lay where the other chickens lay as it’s seen as a safe place so that probably applies to nest sites too. They see it as a safe place. I have multiple nest boxes for mine and they all lay in the same one or two and yes sometimes have been in there together.
They could also be lower on the pecking order and be hiding from the others in there. Although if the egg production has dropped then I bet it’s probably broody instead and they are probably chasing the other hens away from the boxes. Broodies can be very ferocious hahah
The other option is your birds may be hiding eggs somewhere if they are free ranging and you may have to go look for them.
This is all normal behavior from them though and they’re not in love! Although they MAY be good friends. My birds tend to break off into various friend groups when they’re foraging for the day. They are one flock but have their best friends just like we do.
Breaking them might prove a bit challenging given they have been broody so long but it might not be.
You could also try putting them on the roost bars at night in lieu of the cage but that may take longer and may not work.
Good luck!