Hinduism.... What is the relation between Shiva and Vishnu ?

One is the protector God while the other is the destroyer God. But in certain myths they do interchange roles. How is the karma of both defined? Do they act complementary to each other of against each other? How is their role perceived? Your inputs please.

Shiva is just a designation. All fathoms have same relationship with Vishnu irrespective of designations

There is nothing to misunderstanding in it dear!

God is supreme power and none is more powerful than God in the universe so if once something is created by God than how anybody another can destroy that.

so, it only can than possible when it is only one thing

Shiva and Vishnu are both the names of only one God and there nothing means to any destroyer of the word Shiv

Vishnu (Vishr - to pervade) is One who pervades all the universe, animate and inanimate.
and
Shiva is One who is all Bliss and he Giver of happiness to all.

Shiva & Vishnu is one & the same, each portrays different expressions at different times. I believe its similar to how a person can have different various aspects to their character.

Vishnu is "Hari" and Shiva is "Har".

They are homosexuals
Both created Lord Iyappa

If Lord Vishnu is Milk
Lord Shiva is Curd.

Milk can become curd, but curd cannot become milk.

Lord Vishnu becomes Shiva, but Shiva in turn does not become Vishnu.

See the esoteric details below.

The water of the Ganges is supposed to rest on the head of Lord Siva. Lord Siva is an incarnation of the Supreme Personality of Godhead, who sustains the entire universe by different potency's. Lord Siva is described in the Brahma-smite (5.45):
"Milk changes into yogurt when mixed with a yogurt culture, but actually yogurt is constitutionally nothing but milk. Similarly, Govinda, the Supreme Personality of Godhead, assumes the form of Lord Siva for the special purpose of material transactions. I offer my obeisances at Lord Govinda's lotus feet." Lord Siva is the Supreme Personality of Godhead in the same sense that yogurt is also milk although at the same time it is not milk. For the maintenance of the material world there are three incarnations -- Brahma, Visnu and Mahesvara (Lord Siva). Lord Siva is Visnu in an incarnation for the mode of ignorance. The material world exists predominantly in the mode of ignorance. Therefore Lord Siva is compared here to the longitude and latitude of the entire universe, which resembles a cloth woven of threads extending for both its length and breadth.
http://vedabase.net/sb/9/9/7/en2

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 Sambhu is a transformation for the performance of the work of destruction.
PURPORT
(The real nature of Sambhu, the presiding deity of Mahesa-dhama, is described.) Sambhu is not a second Godhead other than Krsna. Those, who entertain such discriminating sentiment, commit a great offense against the Supreme Lord. The supremacy of Sambhu is subservient to that of Govinda; hence they are not really different from each other. The nondistinction is established by the fact that just as milk treated with acid turns into curd so Godhead becomes a subservient when He Himself attains a distinct personality by the addition of a particular element of adulteration. This personality has no independent initiative. The said adulterating principle is constituted of a combination of the stupefying quality of the deluding energy, the quality of nonplenitude of the marginal potency and a slight degree of the ecstatic-cum-cognitive principle of the plenary spiritual potency. This specifically adulterated reflection of the principle of the subjective portion of the Divinity is Sadasiva, in the form of the effulgent masculine-symbol-god Sambhu from whom Rudradeva is manifested. In the work of mundane creation as the material cause, in the work of preservation by the destruction of sundry asuras and in the work of destruction to conduct the whole operation, Govinda manifests Himself as guna-avatara in the form of Sambhu who is the separated portion of Govinda imbued with the principle of His subjective plenary portion. The personality of the destructive principle in the form of time has been identified with that of Sambhu by scriptural evidences that have been adduced in the commentary. The purport of the Bhagavata slokas, viz., vaisnavanam yatha sambhuh, etc., is that Sambhu, in pursuance of the will of Govinda, works in union with his consort Durgadevi by his own time energy. He teaches pious duties (dharma) as stepping-stones to the attainment of spiritual service in the various tantra-sastras, etc., suitable for jivas in different grades of the conditional existence. In obedience to the will of Govinda, Sambhu maintains and fosters the religion of pure devotion by preaching the cult of illusionism (Mayavada) and the speculative agama-sastras. The fifty attributes of individual souls are manifest in a far vaster measure in Sambhu and five additional attributes not attainable by jivas are also partly found in him. So Sambhu cannot be called a jiva. He is the lord of jiva but yet partakes of the nature of a separated portion of Govinda.
http://vedabase.net/bs/5/45/en2

Please chant HARE KRISHNA and be happy.
hare krishna hare krishna
krishna krishna hare hare
hare rama hare rama
rama rama hare hare

Different gods in Hinduism are symbolical.

Brahma- The force that creates the universe
VIshnu- The force that maintains the universe.
Shiva-The force that will cause the dissolution of the universe

Similarly other gods have there own symbolic meanings. Most of the stories in Purana's are symbolic.

To make common man understand all these characters were created, so that humans could respect nature.