When you are trying to detect them at the same time, do you intend to combine (perhaps in a 1:1 ratio) the CellROX and the MitoSOX before adding to the cells?
They may add them separately because they may react with one another.
mitosox excitation is around the emission wavelength of cellrox green (~520), so i m not sure if it is wise to combine them as a part of the latter emitted energy would be absorbed by mitosox, impacting your results.