I've never read or played HotDQ, so I don't know the context of these encounters. Are these both mandatory combat encounters (you cannot progress the adventure without defeating these monsters)? Can the PCs learn about these creatures beforehand and therefore prepare for the encounter? Are there any items in the adventure (a sword of dragon slaying perhaps) that would give them a special advantage in the encounter? All of these are factors.
Furthermore, it does nothing to change the fact that the DMG advises caution with regard to using monsters whose CR is above that of the party.
You know what they say, almost doesn't count.