Part-25 does not say anything about navigation equipment, as that relates to the type of operation rather than airworthiness. If you look in 121.349 for FAA and AMC2 CAT.IDE.A.345 for EASA, you find that in fact only one ILS receiver is needed. And there is no reference to any requirement to have it displayed on the standby attitude. Which is just as well, since there are plenty of Part25 aircraft flying around out there with older standby instruments that have no such display. It's just Mr. Boeing being nice to you!