Typically any geographic area is ruled by the government of one country. Occasionally the idea of shared sovereignty comes up as a solution for disputed areas. For it to work, the agreement needs to specify clearly which government controls what. And the agreement needs a mechanism for resolving disputes. It's not unlike a joint venture between two private sector companies - they need to agree on who is responsible for what, and how to resolve disputes.
There are a few cases now of postage-stamp countries whose foreign affairs are controlled by a big neighbor (eg San Marino and Italy, Monaco and France). The heads of state of Andorra are the Bishop of Urgel, Spain and the President of France - I suspect that Spain or France controls Andorra's foreign affairs.
The Temple Mount in Jerusalem is under the control of the Muslim wafq, but Israel retains control of security. Joint sovereignty has been proposed for all of East Jerusalem as part of a peace agreement between Israel and Palestine.
Written 17 Sep 2015 • View Upvotes • Asked to answer by Abhimanyu Singh