Depends on your training volume. If your considering it then I would guess that youve done both your training and your research on the idea. Can it be done, obsolutely, is it something you want to do, thats up to you, but eight weeks is more then enough time to recover and taper in to a HIM. Good luck!

Depends on your experience level too. If this marathon is just another notch in your belt and the HIM presents no new challenges, then sure. Then again, if that was the case, you probably wouldn't be posting this question. So then eight weeks does sound a bit close if you are shooting for optimal performance in a marathon and HIM. For example, 2 weeks post marathon recovery plus one week HIM taper leaves you five of the eight weeks for HIM training between the two events. Now if you have a long preseason with a significant SBR base, then those five weeks of usuable time might be enough. I might chose one to push and to have fun at.

Depends on the marathon and if you're prepared to run it. Then it depends on your goals for both. I'd say generally that they're far enough apart to set some aggressive event targets. If you're following a periodized training plan, you might have some cross-interference with planned workouts if you've separated the two events. Watch out if you have two heavy training weeks back to back, make sure you plan in recovery time.

Cheers,
-Brandon