Skanda Upanishad belongs to Krishna Yajurveda. Here you can download Skanda Upanishad PDF document for free. It explains how and why the Shaivism and Vaishnavism are similar and the same. It concludes saying both are same and united.
Skanda Upanishad also tells about Lord Skanda (Murugan / Kumara swamy / Subramanya swami).