I don't advocate two turtles in one tank, I just think it is too big of a risk. If you have a large pond, that's one thing, but having two turtles in one tank seems like a notorious recipe for aggression. Keep in mind that turtles are very solitary and don't require a "friend," they see it more as a competitor.
That being said, occasionally two turtles do seem to live together well. It may not last forever though, so you do need to be prepared to have two 100 gallon tanks for two adult turtles if problems arise. If that is doable for you, then by all means get two turtles, if it isn't something you can or want to do, then stick with one.
If you're set on two turtles why not investigate some less aggressive breeds like the painteds.
The nice thing about not having the turtles yet is having all the time in the world to research what will work best for you.
