sorry to hear about the loss. that's sometimes tricky when having monogamously paired butterflies.
the other one lags behind.
in fact, mertensii and other species of the "xanthurus" complex do not always form male/female bonds in the wild. there are instances where male-male pairs are formed and they together, protect a territory.
i believe this goes for other species.
no idea how it would impact the social and feeding response in the tank though.