I’m riding in an AIDS fundraiser ride in about 2 months that’s two days long, 100 miles each day. I’ve ridden 100 mile rides before and many back-to-back 60-70 mile rides, but never two 100 milers back-to-back.
I’m in a SYSTM training program and complimenting it with road rides. Any recommendations on how to make it through two 100 mile days back-to-back?
Thank you.
Don’t stop pedaling 
Seriously though, as with any endurance event, make sure your hydration and nutrition are dialled in.
I have ZERO doubt that if you’ve done many 60-70 back-to-back rides you’ll be quite capable of a 2x100. The profile of the course and pacing will play a bit of a role so you don’t want to burn ALL of your matches on day 1 such that day 2 is nothing but a slog (but it will probably be a bit of a slog anyway 
).
If you’re not riding alone, take advantage of the draft when it’s available to you and enjoy the ride!
Here’s a thought for you, one that works for me to stave off any doubts that I have.
“Finishing is never in doubt, the only question is how long it will take.”
You’ve already got this, it’s as good as done, because you have resolved to do it. As Sir @Glen.Coutts says, “don’t stop pedaling.”
He also gives good advice on pacing and having your hydration and nutrition dialed in. If you’ve done 100 milers, you’ll already have nutrition figured out.
The only additional thing I would add is to manage your post-ride recovery well.
Mostly what everyone has said. Elevation will be the main determination of how much it will hurt.
Eat often, eat early. Don’t wait until you need to eat, and when you finish day 1, get a shower and a good stretch out, especially the back and shoulders, then refuel properly, but nothing too fancy that may cause ‘gastric distress’ in the morning 
Have you included the mental training in your training plan? If not it would be worth going through some of it as that will help. There’s at least one podcast (The Knowledge) on setting goals which may help too.
All the very best for your challenge! I’m sure you’ll crush it.
I LOVE the article By The Numbers: Ian Boswell’s Unbound Win for the in depth analysis of how quality metabolic training was key for Boswell’s success. There is also great information on fueling strategies. Another great article concerning energy expenditure and fueling is the article By the Numbers: What it Takes to Race a Grand Tour.
If you want some individualized advice you can utilize a One-on-One Call with a Wahoo Coach
P.S. in addition to the SYSTM Mental Training program, I also highly recommend the Wahoo SYSTM Strength program for increased mobility, stability, muscle requirement, balance, and strength. It really is a brilliant program, and I cannot recommend it enough.