A strong Hive can ward off parasites such as wax moth or SHB where a weak one can't. In your case, if I were doint it, I'd buy a new queen and combine the two weak hives. That way if there is a wax moth outbreak the stronger hives are not contaminated, the combined hive has a better chance of survival, and the loss, if there is one, is limited to the 2 hives that were weak and endangered to begin with.