In the Alpha dominated First Order, Omegas have no rights or use beyond breeding. Both Kylo Ren and Hux have somehow managed to hide their omega natures and rise to their current positions. That is until they meet, and immediately know that not only is the person they are looking at an Omega in hiding, the other person has recognized they are one too. They both have this secret to hold over each other, and have to keep the others secret if they want to keep themselves safe from being chained up in the breeding pits.
Of course they despise each other. But they can't risk letting the other out of their sight, so they are stuck, locked in each other's orbit, alternately protecting each other's secret and plotting to out the other while keeping their own secret.
And during heats, well. There's really no one else they can turn to except each other now is there.
TLDR: Ren and Hux, secret Omegas, trapped into protecting each other's secret and coincidentally having truly epic Omega/Omega hatesex.
Omegaverse AU: Kylo Ren and General Hux, secret Omegas.
Of course they despise each other. But they can't risk letting the other out of their sight, so they are stuck, locked in each other's orbit, alternately protecting each other's secret and plotting to out the other while keeping their own secret.
And during heats, well. There's really no one else they can turn to except each other now is there.
TLDR: Ren and Hux, secret Omegas, trapped into protecting each other's secret and coincidentally having truly epic Omega/Omega hatesex.