To add one further point to the previous post, Tosfos quotes Shabbos 25 which prohibits lighting oil of terumah t’meiah on Shabbos or Yom Tov. Even though burning kodshim for disposal is a mitzvas aseh, an aseh cannot be doche Yom Tov which is an aseh + a lav. Asks Tosfos: why can’t women, who are not obligated in the aseh of Yom Tov because it is zman gerama, use this oil for hadlakas neiros – since they are only obligated in the lav of Yom Tov, we should invoke aseh doche lo ta’aseh. Tosfos’ question clearly assumes that the aseh and lav or Yom Tov are treated as separate independent elements.
Tofos answers that since the lav of Yom Tov is conjoined to an aseh, it indicates (is a siman) that this lav is stronger than other lavim and cannot be pushed off by an aseh. Not quite the sevara of the Ramban, but close. According to Ramban, the answer is much simpler: since the aseh is associated with a lav, the two function as a single unit. Since women are always obligated in the lav of Yom Tov, the aseh always tags along as well.