Some lucky retirees split their time between two different states. Legally, you do not need separate estate planning documents for each state, but it may make sense from a practical perspective.
The Constitution of the United States requires that states give “full faith and credit” to the laws of other states. This means that your will, trust, durable power of attorney, and health care proxy executed in Tennessee (just to pick one state) should be honored in Alaska, Arizona, California, Colorado, Oregon, and every other state. That's the law.
The practical realities, however, are not the same as the legal requirements. While you should not need a separate will or trust for a second state, your power of attorney and health care proxy may be a different story. Financial and health care institutions are used to the documents used in their states and may refuse to honor out-of-state documents. In the case of health care proxies, other states may use different terms for the document, such as “durable power of attorney for health care” or “advance directive.” (And the people reviewing your power of attorney or health care proxy may not be well versed in constitutional law.)
In the absence of a durable power of attorney or health care proxy, family members often must resort to going to court to be appointed guardian or conservator. This causes delay and expensive and unnecessary legal fees. So, even though it should not be necessary, if you do spend a good part of the year out-of-state, executing a local health care directive and durable power of attorney may be a good idea. Make sure that you appoint the same people you appointed on your other estate planning documents, so that there's no confusion about who should act for you when the time comes.