It just occured to me too! Given how the maids keep saying that it was a good timing that Eric and Miesa are getting along very well. They knew they were intimate and they knew they will kill two birds with one stone with their plan, assuming it would be a successful one, anyway.
....andd the reason Eric did not thought of that was because he was too worried about Miesa that he couldn't really set his mind straight at the moment.
ur smarter than me fr i didnt think of that
I feel like the plan was not simply poisoning her, but to poison him. The best way was through kissing. How is the smart ml not getting that?? Or is it lost cuz if the translation?