There is no way I can get by with only two bottles of water. I live in Southern California and I primarily do long climbs. The biggest climbs right near me are only (Strava) Cat 2, but I like to head out for climbs on Cat HCs. No stores on mountain climbs.

When I tackle Mt. Wilson, I carry a disposable bottle of water in one of my jersey pockets. I will drink this bottle until I arrive at the actual base (but there still is plenty of climbing until that point). I will then throw the bottle away and use my two bottles on my bike to get to the first range station, where I can get more water. Hydration is key. 100k with only two bottles?

I guess it does get hotter in Southern California :) 90°F days are rare here, and there isn't much in terms of cat 2 or HC climbs either, just lot's of rolling terrain (100k will average out to 1000m vertical, 2000m if you seek out climbs).