When you say 'good' in both distances, what is your definition of good?
I think it's entirely possible to be good at both since they are both endurance based events albeit that the 800m requires far more speed.
I think by getting up around 50 - 60 mpw will be sufficient for your marathon but for your 800m, you will need lots of quality workouts with the emphasis on speed - so 100m, 200m & 400m reps are king here.
In addition, I would throw in strides at the end of every 'easy' long run to just remind your legs that they need to run quickly for the 800m.
Ultimately, multi distance calls for multi paced training.