The following two verses are solutions to a single challenge that posits Viṣṇu is Śiva:
कपाली च कलापी च शिरसा भाति योऽवतात्।
स देहार्धसमासक्तकेशवश्चन्द्रशेखरः॥ (Shankar Rajaraman)
May Śiva who wears a skull as his crest-jewel and Kṛṣṇa who wears peacock feather in his diadem protect us in the form of Hari-hara. In this undivided manner, Viṣṇu is indeed Candra-śekhara.