When I did my first couple of two day double centuries, I remember feeling like there was no way I was getting back on the bike after day one. I was toast! But, surprisingly enough after some good food and a nights rest I felt super strong the next day and finished feeling great. If you have an option to go a little farther the first day than 100 miles, it is a nice mental boost to know you have less than 100 to do the second day even if it is only 10 miles or so.

As long as you train, you will be fine. The more training you get in the more enjoyable the ride will be.