We recommend waiting until the first coat is fully cured. It is best to give it 24 hours so that all areas, including the hairline cracks that get filled, are fully dry. If the driveway has been sealed early in the morning and the entire driveway is fully dry, you may put a second coat on later in the day.
Applying two coats of Super Seal TM will dry and cure much better than one thick coat. All Super Seal TM products are water based, which "cure" via evaporation. A thin coat will evaporate much faster than a thick coat will. If you apply one thick coat, you are "locking in" that initial coat and the sealer will hold water which causes tracking.