It depends on the athlete and their goals. The fitter, more experienced, and more durable the athlete, the less time they need between 70.3’s to perform well in the second of the two. If the first 70.3 is an “A” race and the second is just for kicks, it really doesn’t matter how closely spaced they are. If they’re both “A” races, they’ll require more separation. If the first 70.3 is a “B” race or just for kicks and the second is an “A” race, they can be a little closer together than if they’re both “A” races but not as close as if the second race is just for kicks.