Double Life (2024)

the idea of poison and transfer of it was very creatively used in Butterflied Lover. Double Life is not about the same thing but uses the main character's dilemma with the poisoned beloved...

Both have a similar plot regarding the FML being an assassin/spy and infiltrating the palace to complete their mission.

This is not exactly the same as being married to one, but loving a female assassin while having some hidden agenda on his own...

Again, not married to someone with a hidden agenda, but successfully deceiving each other about true identity or purpose