With such varying demands between the two races, there’s no way you can perform at the optimal level for both, which means you’ll need to compromise. Since the 100mi epic sounds the most physically demanding, I’d focus on training for that event. You don’t want to be completely destroyed afterward and unable to race the enduro.
Now, as an avid enduro racer, my advice would be to simultaneously train your descending skills. While training with TR for your epic, you can still ride outside (especially chairlift/shuttle days) and practice in the dirt. Ultimately, if you’re in enough shape to complete the earlier epic, you should have no problem riding the transfers/liaisons between stages - but the race is won and lost on the race stages. From what I’ve seen regarding the Ard Rock, you’ll have to deal with plenty of loose rocks, dry and dusty conditions, and high speed sections. There’s also some tight switchbacks, but I think traction will be your greatest enemy.
The best way to get comfortable with riding the edge of traction is to ride more, so I’d recommend making all your outdoor rides focus on descending more aggressively and faster, while using your trainer sessions to get you ready for the longer race. Hope this helps!