There have been many examples of countries being ruled separately by the same monarch. The British monarch was also the ruler of Hannover in Germany from 1714-1837, and the present Queen Elizabeth II is also queen of many other countries such as Australia. There is no expectation that countries ruled under a personal union should be joined together as England and Scotland were in the 1707 Act of Union.

u/deleted has a relevant answer here: https://www.reddit.com/r/AskHistorians/comments/fiqhe3/comment/fkjgkvk

u/wishbeaunash talks a bit about how Scotland was governed during the period of personal union here: https://www.reddit.com/r/AskHistorians/comments/fzobd3/comment/fn6djfl

As for why the Act of Union happened in 1707, a number of users have answers in this older thread: https://www.reddit.com/r/AskHistorians/comments/1xssak/what_was_the_popular_opinion_of_the_act_of_union/

There is a lot more that can be said, so hopefully someone else will be able to offer their own perspective and expertise.