In India in many places, Lord Siva is worshipped on equal platform with Lord Vishnu or Narayana. Technically, assuming both Lord Siva and Vishnu as same is an impersonal understanding of the Vedas. If Siva and Vishnu exist simultaneously and differently, then surely there has to be a personal reason for it...otherwise what is the need for two different manifestations. However, to be politically correct or spiritually lazy to understand deeper or due to sentiment, the general populace and even learned scholars equate Siva with Vishnu on the same platform.
Actually, they are same but at the same time not same. The actual truth is that Vishnu is Siva in the material sense. Vishnu who is completely spiritual and pure when in contact with material nature appears as Lord Siva. Therefore it is Lord Vishnu who transforms into Siva for the sake of creation and not vice versa.
This is clearly explained in the Srimad Bhagavata purana and Brahma Samhita. Please see below.
Actually Lord Viṣṇu maintains and accomplishes all good fortune. If one has to take shelter of Lord Viṣṇu, why should the demigods take shelter of Lord Śiva? They did so because Lord Viṣṇu acts through Lord Śiva in the creation of the material world. Lord Śiva acts on behalf of Lord Viṣṇu. When the Lord says in Bhagavad-gītā (14.4) that He is the father of all living entities (ahaḿ bīja-pradaḥ pitā), this refers to actions performed by Lord Viṣṇu through Lord Śiva. Lord Viṣṇu is always unattached to material activities, and when material activities are to be performed, Lord Viṣṇu performs them through Lord Śiva. Lord Śiva is therefore worshiped on the level of Lord Viṣṇu. When Lord Viṣṇu is untouched by the external energy He is Lord Viṣṇu, but when He is in touch with the external energy, He appears in His feature as Lord Śiva.
Just as milk is transformed into curd by the action of acids, but yet the effect curd is neither same as, nor different from, its cause, viz., milk, so I adore the primeval Lord Govinda of whom the state of Śambhu is a transformation for the performance of the work of destruction.
Sri Brahma-Samhita 5.45
As a note, there may be other puranas and upa-vedas that may extol Lord Siva as the Supreme Lord, father, paramatma and controller of all that be...that is again in the context of within this material creation and not beyond. But the above purports from Bhagavatam and BrahmaSamhita talk about a transformation from spiritual to material and not solely within material activities. Because Lord Siva is seen in the context of within material activities, He is considered as a powerful Demigod (Maha Eashwar or Maheshwar or Great Demigod). However, the goal of human life is to break the cycle of birth and death and go back to the spiritual world and for that to happen, the only way is to surrender to Narayana (not Siva) for He is beyond material activities and is fully spiritual. Therefore Narayana is addressed by great saints as the Supreme Absolute Truth (svayam Bhagavan) or Narayana paro avyaktat (Narayana transcendental to material manifestation)