Complex sentence: Most of the sand-skiing in Namibia is performed in the Namib desert dunes around Swakopmund and Walvis Bay, however, with a special permit it is sometimes possible to sand-ski at the world's highest dunes in Sossusvlei.
Split into simpler sentences: Most of the sand-skiing is performed in the Namib desert dunes around Swakopmund and Walvis Bay. With a special permit it is sometimes possible to sand-ski at the world's highest dunes in Sossusvlei.