When a royal heir marry another royal heir....Do the two country's become one in reign to the Royal coupling?

Yes, provided the royal couple has children to inherit the two countries together. This is how Spain came to be unified, with the marriage of Ferdinand of Aragon and Isabella of Castille. Moreover, The UK and Hanover were ruled by the same monarch until Queen Victoria's reign (Hanover did not allow women rulers).

When two monarchs marry, their kingdoms unite with each monarch ruling their country individually. However, after their deaths the two countries are ruled under one monarch (their heir) and remain so as long as there is an heir who can rule both countries together. Each country has its own laws but share the same monarch.

Queen Mary I and King Phillip of Spain never had children, so upon the death of Mary her kingdom went to her sister Elizabeth, who was next in line to the throne. Philip quite wrongly believed he had a claim over England as Mary's husband, which he would have had he sired an heir with Mary. But he did not.

No. The King of Spain and the Queen of England were married - the countries remained separate and it eventually resulted in the so-called Anglo-Spanish War.

No. Each reigns their own realm.

Yes. For example, if Albert of Monaco had married Victoria of Sweden, a new country called Moneden (or Swonaco) would have been formed.

No.

In 1603 King James VI of Scotland inherited the throne of England from his cousin Elizabeth I, but the two countries remained separate, with borders, customs duties and the whole deal, till 1707 when the Scottish government bankrupted the country and to get Scotland out of this fix the two countries agreed to become a United Kingdom by passing an Act of Union in each of their respective parliaments. If the Scots had managed their own affairs better, the two countries might never have united.

Then in 1714 the Elector (hereditary ruler) George of Hanover inherited the new United Kingdom from Queen Anne, and he and his descendants ruled the two separate realms of the UK and Hanover until 1837, when Victoria succeeded to the throne of the UK. But she couldn't succeed to the Hanoverian throne because Hanover had the Salic Law which excluded women from the succession; so a cousin inherited Hanover and the two countries went their separate ways again.

yes but then they would be very hairy and strange.

No, they become close allies.

When Queen Mary was married to the King of Spain her husband Philip II of Spain had his eyes on the ownership and rein of England. After Queen Mary died Princess Elizabeth (Queen Mary's sister) came to the thrown and became Queen Elizabeth. This caused Philip II to launched an attack on England but England won that war, and Queen Elizabeth remained Queen untill she passed away of old age.

https://en.wikipedia.org/wiki/Anglo-Spanish_War_(1585–1604)#English_Armada

Makes them pretty hairy