To answer this question. We have to go into Saiva Sithaandham. Firstly, there are 3 terms to note. Padhi (God), Pasu (soul) and Paasam (Bondage). Pasu experiences both Paasam and Padhi. Paasam doesn't experience as it is unintelligent. Padhi is all Intelligent hence there is no need for experience. Padhi is attributed with 8 Natures. An entity must satisfy all 8 Natures to be considered as Padhi. Given this, Padhi has no reason to meditate as meditation is used for gaining clarity. If Padhi is Padhi then there is no need for Padhi, Who is Omniscient, to meditate. However, there's a form seen in meditation, accepted as Lord Sivan, in meditation. There is an initiation called Saaroobam. Saaroobam is where the soul receives ornaments of the Ruler. Those Who revere Lord Sivan and Who are bestowed with this initiation, will resemble Him to a certain extent. This initiation is usually given to rulers of other worlds directly below Sivalohgam or to look over a certain group of worls. These Rulers are know as Rudhraas. So, truly, that which people see in meditation that looks similar to Sivan wrongfully concludes this to be Sivan but it is actually Rudhran. We may argue this but, there are so many Who worship other images as Sivan which is not according to scriptural dsescription.
Let us go to one of the first Historical events concerning Lord Sivan, VishNu and Bramma -- Thiru Kaarthigai Dheebam. Here, we find VishNu and Bramma in an argument as to whom of the two are greater. Immediately, we see ego and arrogance which are characteristics of souls and not of God. A Light (Sivan) Manifests between both. Neither VishNu nor Bramma found the beginning or end. VishNu accepted this and rose chanting the AindheZHuthu (Panchaakshara) whilst Bramma came across the ThaaZHambu flower and brought it as proof trying to lie that he had founds the Head of Sivan. Doubts as to whether this event took place or not is easily cleared by archaeological finds and current spiritual practises. After surrending to the Light, VishNu was blessed with 108 temples in His name. His devotion and honesty is the reason as to why VishNu is respected by Saivites. Bramma hasn't more than 5 temples in his name. Thiru ANNaamalai was left when the Light disappeared. Today, Kaarthigai Dheebam is about the biggest religious event in South India.
We come to the idea of incarnation. Birth causes misery and God's duty is to relieve us this. Now, for an entity to completely relieve us of something, that entity cannot be affected by it therefore, God does not take birth (appear via the womb, sweat, egg) of any life form. We can then ask, who are the parents of Lord Sivan? Compare this to asking who the parents are of VishNu? The idea of Hanuman being Sivan is also untrue and isn't supported by scriptures. There are numerous Saiva ThalangaL (temples where Tamil Saiva Saints sang) have records of Hanuman worshipping Sivan. Besides, Raamaa worshiping at Thiru Viraamehchuram (Raamehshvaram), He worshiped many Sivalingams and installed as well, throught his journey. Only when he performed Dhavasam for Dhasarath and Thiru Thillaidhaipadhi, in Worship to Lord Sivan, did Dhasarath attain Muthi.
When KrishNa performed archanai to the Sivalingam, as he wasn't initiated to perform Siva Punniyam (Poosai), Ubamanyu Munivar took those flowers away. KrishNa then approached Him and Ubamanyu initiated him into Siva PuNNiyam. I read here that KrishNa gave the 1008 names of Sivan. That itself is Chanting of Maha Mantra. KrishNa gave ArjuNaa Vishvaroobam only because He was experiencing Sivameh Yaam (Sivohgam Baavanai/Thathvamasi). This is when the soul experiences Padha Muthi and has surrendered its identity unto that of Lord Sivan and identifies only with that. Therefore, a soul, in that state, will say, "I am Sivan. I am God. I am the universe". The universe is not Sivan and vice versa but Sivan is omnipresent within and exists as the universe for the universe to exist.
The Siva-VishNu Form: when VishNu was been troubled by Salandhran Asooran, he performed worship to Sivan (Podhu Siva PuNNiyam) for assistance using 1000 Lotus flowers. Sivan made one go missing. VishNu then plucked out an eye and offered as once Siva Poosai starts, you can't wake up until it is over. This is when Sivan gave the Sakkaram. As this was a weapon given by Sivan, this weapon is Sathi (Divine Grace). The power of this was too intense for VishNu. So, Sivan Manifested Himself on half of VishNu for control of the Sakkaram. Philosophically, this represents the Purusha Thathuva where the Vidhya thathuvas are joined with the soul and exist as the soul in order for the soul's powers to function. It also represents the Adwaitha (Non-Dual) relationship that exists between the Lord and souls where the Lord has to exist as one as the soul, different from the soul and together with the soul, simulataneously, in order for the soul to exist and function, be driven into the experience of the world and in order to know and experience the world (universe).
There is also intention. There are two types of Brammaa, VishNu and Rudhra (Hindu Trinity). There is Lord Sivan as Maha Brammaa (Instructor Creator), Maha VishNu (Instructor Preserver) and Maha Rudhraa (Instructor Disolver). This is Only ONE. Then, in order to create, preserve and dissolve the many worlds to cater for the many different souls, there are souls who are given these duties. These become Brammaas (Instructed creators), VishNus (Instructed preservers) and Rudhraas (instructed dissolvers). Each division of a universe will have its own set and there are millions of universes. The 3 dsivisions are the Aanma Thathuva worlds, the Vithya Thathuva worlds and the Siva Thathuva worlds. The Bramma, VishNu and Rudhra that are popular on earth are the creator, preserver and dissolver of the Aanma Thathuvas which our world belongs to. Heading this world is Rudhraa known as SeekaNDa Paraman. He represents Sathwik (being in meditation) whilst VishNu represents Thamasik (seen sleeping/lying down) and Bramma represents Rajasic (4 heads, crowns). Spiritually, then, Rudhraa is most superior. He only comes out of Siva Dhyaanam to perform the duty He is given and returns. So, Sivan can be worshiped as Maha VishNu but, this will be different from the instructed creator of our worlds who is attached to dhasavadhaaram. At the same time, Sivan exists in a non-dual state with this Vishnu (who is a soul) and makes him assume these avadhaarams. So, Sivan can be worshiped through these avadhaaram as the One Who Ultimately Instructs and Decides.
Therefore, to give VishNu an equal or high status to Sivan, given authentic scriptures, practices and archaelogical evidence, goes even against VishNu who idolised and worshiped Lord Sivan as his Lord and Guru.