sounds like a great plan to me. leave the 3724 at 85*F for 10 days at least. make sure the 3711 is fully active/at high krausen when you pitch the whole thing. because you're throwing in the starter beer, don't use a stir plate (or only use one for the first few hours, turn it off before fermentation is visible).I might try pitching 3724 at a high temp (85F) for saison flavor, then cooling to 70F and pitching Belle saison to get good attenuation and body.
be prepared for an excellent saison