Based on the wording of the rules I would say Yes, you get both effects from a single cast.
Under normal circumstances a single cast of a Restoration spell (or the Lesser/Greater variants) can dispel harmful magical effects or cure ability damage but it will still eliminate fatigue regardless of which you choose.
Since the rules on sanity do not explicitly say you must choose to cure sanity damage or reduce the DC I would interpret that you receive both beneficial effects, as well as the normal effects of the spell.
Sanity is an optional rule and you (or your GM) will need to decide to what degree it is implemented and how oppressive it will be. If you play with these rules and find that it makes restoring sanity damage and madnesses too easy, you can always tweak it for your own campaign.
If you read the Sidebars on the Sanity rules page they give you some ideas for possible tweaks that can be made to the system in order to customize it for your game.
In a particularly horror-themed game, the GM may consider removing some or all of the magical options to reduce sanity damage, relying on rest alone to recover sanity.
If you want to feature the madnesses as afflictions that are simple to remove for high-level spellcasters while outside the reach of anyone else—as per the more baseline experience—assume lesser madnesses are not affected by lesser restoration, and greater restoration, heal, and psychic surgery remove all madnesses, whether lesser or greater.