Shri Sanatanam: HariHara

To whom the title of Maheshwara is given, Lord Shiva or Lord Vishnu ?

My whole answer will be based on Suddha Adwaitam Shaiva Siddhanta and Adwaita Vedanta too which I firmly believe and follow !!!!!!

Maheshwara is a term can be used for both Lord Shiva and Lord Vishnu but predominantly it's used for Lord Shiva only as his another name and it’s very rarely used for Lord Vishnu. Almost every scripture call Shiva as Maheshwara. The word Maheshwara is made up of two words, Maha + Ishwara which means Great God and it’s basically one of the popular names of Lord Shiva like Rudra, Mahadeva, SadaShiva, Sambho, Bholenath, Shankar, Neelakantha, Mahakaal, Bhairava, etc. In Vedas if we deeply observe, Ishwara word mostly comes whenever Lord Shiva’s description or any verse dedicated to him comes up.

Like :-

ओम नमस्ते अस्तु भगवने विश्वेश्वराय महादेव्य त्रयम्बकाय त्रिपुरान्तकाय त्रिकाल अग्निकालाय काल्लग्निरुद्राय नीलकण्ठाय श्री मृत्युंजयाय सर्वेश्वराय सदाशिवाय श्रीमन महादेवाय नमः
“Namasteastu-bhavagan-vishvesvaraya-mahadevayatriyambakaya triupurantakaya trikalagni kalaya kalaagni Rudraya Nilakanthaya-mrutyunjayaya-sarveshvaraya-sadashivaya Sriman mahadevaya Namah”
:- Shri Rudram, Yajur Veda

So here also you might have observed the word “Sarveshwaraya”. This word is made up of two words, Sarva + Ishwara which means Lord of All, whatever it may be, whosoever it may be. Either they be animals, birds, fishes, plants, humans, asuras, bhoot, preta pisaach, yakshas, gandharvas, gods, goddesses, etc. etc. He is God of all.

ईशान सर्वविद्यां, ईश्वर सर्व भूतानाम् ब्रह्म आदिपति ब्राह्मणोआदिपति ब्रह्मा शिवोम अस्थु सदाशिव ओम !!!!!
Ishaan sarva vidyaanaam, Ishwara sarva bhootaanam, Bramhadi pati, Brahmanoadipati Bramha Shivo me astu SadaShiva !!!!!!!!
:- Sadyojadham, Yajur Veda

Again here Lord Shiva is called as Ishwara. So basically Ishwara term in Vedas too is used for Lord Shiva only.

Now coming to the word Maheshwara.

In the Shiva Ashtottara Shatanamavali (108 names of Lord Shiva) composed by Lord Shri Narsimmha Swamy (Lord Narsimmha Narayana Bhagwaan) and it comes from Shri Linga Purana, also mentions it.

The second name only, see this :-

ॐ शिवाय नमः
ॐ महेश्वराय नमः
ॐ शंभवे नमः
ॐ पिनाकिने नमः
ॐ शशिशेखराय नमः
ॐ वामदेवाय नमः
ॐ विरूपाक्षाय नमः
ॐ कपर्दिने नमः
ॐ नीललोहिताय नमः
ॐ शंकराय नमः
Om Shivaya namaha
Om Mahe-shwaraya namaha
Om Shambhave namaha
Om Pinaakine namaha
Om Sasi-shekha-raya namaha
Om Vama-devaya namaha
Om Virupakshaya namaha
Om Kapardhine namaha
Om Nila-lohitaya namaha
Om Shankaraya namaha
:- Shri Linga Maha-Purana

These names were composed by Lord Vishnu in his Narsimmha Avatar when Lord Shiva as Lord Sharbha was trying to pacify Lord Narsimmha by tightly holding him. The story is 100 % Authentic because it’s mentioned in Upanishads too which makes it 100% Vedic and hence Shri Shiva Ashtottara Shatanamavali too is 100% Vedic.

Now coming to Shiva Purana, in Shiva Sahasranama Stotram which was composed by Lord Shriman Narayana himself. It starts with Dhyaanam :-

Om Dhyaye Nityam Mahesham Rajatagirinibham Charuchandraavatamsam
Ratnakalpa Jwaalaangam Parasu Mriga Vara Bheetihastam Prasannam

Here too the word Mahesham or Maheshwara comes.

Now when we go in Shiva Sahasranama, in the 19th verse, again the word Maheshwara comes.

कालयोगी महानादः सर्ववासश्चतुष्पथः ।
निशाचरः प्रेतचारी भूतचारी महेश्वरः
Kaalayogii Mahaanaadah Sarvavaasashchatushhpathah
Nishaacharah Pretachaarii Bhuutachaarii Maheshvarah

So even here too Lord Shiva is called as Maheshwara !!!!!!!!

Even we have different stotras which proclaim them as supreme like Uma Maheshwara Stotram, Shivashtakam, Vedasaar Shivasatvah Stotram, Vishwanathasktakam and many many times.

Even Maheshwara Kavacham is dedicated to Lord Shiva only. Even in Mahabharata, Pandavas use to worship Lord Shiva by calling him Maheshwara many times.

However the term is lessly given to Lord Vishnu but some synonyms of that term (Maheshwara) is given to Lord Vishnu like we see in Shri Vishnu Stotram in Shri Vishnu Purana.

योऽन्तस्तिष्ठन्नशेषस्य पश्यतीशः शुभाशुभम् ।
तं सर्वसाक्षिणं विश्वं नमस्ये परमेश्वरम्
Yoh-ntstisthannseshaya pashyantisha shubhashubham
Tam sarvasakhidan vishvam namasye parmeshwaram
:- Shri Vishnu Purana

Now coming to Shri Vishnu Sahasranama from Shri Vishnu Purana only we see these names also :-

योगो योग-विदां नेता प्रधान-पुरुषेश्वरः ।
नारसिंह-वपुः श्रीमान केशवः पुरुषोत्तमः ।। 3 ।।
yogo yogavidāṃ netā pradhāna puruśheśvaraḥ |
nārasiṃhavapuḥ śrīmān keśavaḥ puruśhottamaḥ (3)
ईश्वरो विक्रमी धन्वी मेधावी विक्रमः क्रमः ।
अनुत्तमो दुराधर्षः कृतज्ञः कृति: आत्मवान ।। 9 ।।
īśvaro vikramīdhanvī medhāvī vikramaḥ kramaḥ |
anuttamo durādharśhaḥ kṛtajJṇaḥ kṛtirātmavā (9)
महेष्वासो महीभर्ता श्रीनिवासः सतां गतिः ।
अनिरुद्धः सुरानंदो गोविंदो गोविदां-पतिः ।। 20 ।।
maheśvāso mahībhartā śrīnivāsaḥ satāṅgatiḥ |
aniruddhaḥ surānando govindo govidāṃ patiḥ (20)
:- Shri Vishnu Purana

So this is because Lord Vishnu is one of the form of Lord Shiva only and hence their is no difference between Lord Shiva and Lord Vishnu and hence they are one only.

Now how I can say it, see the verse from Yajur Veda :-

शिवाय विष्णु रूपाय शिव रूपाय विष्णवे
शिवस्य हृदयं विष्णुं विष्णोश्च हृदयं शिवः
Shiva is Vishnu and Vishnu is Shiva
Vishnu lives in Shiva’s heart and Shiva lives in Vishnu’s heart !!!!!!!!!!
:- Yajur Veda

See this verse from Shiva Purana :-

“Yatha Jaley Jalam, Kshiptham, Ksheeram, Ksheeray Kruthang Kruthey Yekayeva Thatha Vishnu: Shiva Leetho Nachanyatha”
Be aware that we are both one and the same like water and water, milk and milk, ghee and ghee, both Shiva and Vishnu are inseparable and to be worshiped as one”
:- Shiva Purana

See this verse from Vishnu Purana :-

ब्रह्मत्वे सृजते विश्वं स्थितौ पालयते पुनः ।
रुद्ररूपाय कल्पान्ते नमस्तुभ्यं त्रिमूर्तये ॥३॥
Bramhatvey Srujatey Vishwam Sthiyatou paalayatey namaha,
Rudrarupaaya Kalpaantey Namastubhyam Trimurtaye !!!!!!
Oh Lord you are the one who becomes Bramha and creates Universe, and in your swaroop you preserve it, By becoming Rudra (Shiva) you destroys, so I bow to the holy trinity !!!!!
:- Vishnu Purana

Hence you can see everywhere that Lord Shiva and Lord Vishnu is one as HariHara !!!!!!!!!