Muruga is created from the third eye of Shiva. The five faces of Shiva are Sadyojata, Aghora, Tatpurusham, Ishana and Vamadeva. Where is the sixth face of Shiva to create another face for Subramanya?
Whether SadaShiva has five heads (with sixth Adhomukha head called Nilakantha) or MahaSadaShiva has 25 heads while revealing Agamas or Shankara form has only one head while living in Kailash, it has nothing to do with the six heads of Kartikeya:
Here is how Mahabharat Vana Parva section 224 describes birth of Kartikeya:
Markandeya continued, 'Then Agni, filled with great joy and delight, married Swaha in the guise of Siva, and that lady joyfully cohabiting with him, held the semen virile in her hands. And then she thought within herself that those who would observe her in that disguise in the forest, would cast an unmerited slur upon the conduct of those Brahmana ladies in connection with Agni. Therefore, to prevent this, she should assume the disguise of a bird, and in that state she should more easily get out of the forest.
Markandeya continued, 'Then assuming the disguise of a winged creature, she went out of the forest and reached the White Mountain begirt with clumps of heath and other plants and trees, and guarded by strange seven-headed serpents with poison in their very looks, and abounding with Rakshasas, male and female Pisachas, terrible spirits, and various kinds of birds and animals. That excellent lady quickly ascending a peak of those mountains, threw that semen into a golden lake. And then assuming successively the forms of the wives of the high-souled seven Rishis, she continued to dally with Agni. But on account of the great ascetic merit of Arundhati and her devotion to her husband (Vasishtha), she was unable to assume her form. And, O chief of Kuru's race, the lady Swaha on the first lunar day threw six times into that lake the semen of Agni. And thrown there, it produced a male child endowed with great power. And from the fact of its being regarded by the Rishis as cast off, the child born therefrom came to be called by the name of Skanda. And the child had six faces, twelve ears, as many eyes, hands, and feet, one neck, and one stomach. And it first assumed a form on the second lunar day, and it grew to the size of a little child on the third. And the limbs of Guha were developed on the fourth day. And being surrounded by masses of red clouds flashing forth lightning, it shone like the Sun rising in the midst of a mass of red clouds.
So, Kartikeya has six heads because Swaha (wife of Agni) received seed of Lord Shiva from Agni and she threw it in the lake Six times thus making the born child Six-Headed. Of course it was preordained that Kartikeya would have six heads but it has no relation with the number of Heads of Lord Shiva.
Nama Sivaya Simple fact is the Divinity of Lord Siva's Destruction of Kamadeva, the Fury of the 3rd Eye's Rays from MahaDeva was unstoppable but the impact of Kamadeva [as per Nature Law by Lord Siva self] resulted in the formation of Divine Semen of Lord Siva discharged with the Fury of 3rd Eye of Lord Siva, which needed Agni to hold, until the Saravanna Pogai which had the Blessings to hold the Divine Semen, and had the Six Flowers [some yugas back Divine Ladies were Blessed to bear the Divine Semen, and are waiting], got the Semen as Six Droplets on them, who resumed as Virgin Mothers of Lord Aarumuka and stated to nurture the Lord Himself, further the Six Faced Lord Subramanya [after slaying the mighty demons at Sea, Land and Air]have to stay in this present Kaliyuga with the Sixth Face of Lord Siva [which is called the Progressive Face to the next "Satya Yuga"] with great Penance towards Human and All Beings to take it to the Satya Yuga, and this Sixth Face is All Mercy which holds His Namam only to Protect Life on Universe
The Above Divine Extract is from Lord Siva self, hence Saying Lord Siva's Namam or Lord Muruga's Namam will yield Divinity and Free from Sins witheffect
Lord Sivaya Nama