What exactly do you mean by a "two sided will" as your description is subject to interpretation. If you are merely talking about a will where the provisions are printed on the front and the back of a page then this means nothing at all....however if you are talking about something else then report your question or add additional info so the Avvo attorneys can give you a good answer.
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Ms. Carter gives a good answer. The fact that a will has printing on both sides of the page probably doesn't matter if it meets the other requirements (it is clear it is intended to be a will, it is signed at the end, has witnesses, etc.). The probate office will determine if it is valid. If the will (or more rightly purported will) has an executor named that person should take possession of it if they can and take it, along with a death certificate, for probate. It is usually a good idea for a person to be represented.
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The two-sided part does not worry me. If the will was not supervised by an attorney, that may be a problem because the requirements for a valid will are very specific and technical. One such requirement is two witnesses. There are others. Don't compound the headaches of a do-it-yourself will with a do-it-yourself probate. Take the will to a probate lawyer along with any prior wills, a family tree and a summary of your grandfather's assets. It may only take an hour of the lawyer's time to set you on the right path, or further legal services may be needed.
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If you mean that the will was printed on both sides of each page, that by itself will not invalidate the will.
That, however, is not the whole answer. If the will was printed on both sides, it's a fairly safe bet that it was not supervised by an attorney, because an attorney simply wouldn't do that. By the way, it's also possible that the document you are referring to was a mere photocopy of a will and not the actual will itself.
Take the document to an experienced estates attorney and have him or her review it.
Good luck to you.
Michael S. Haber is a New York attorney. As such, his responses to posted inquiries, such as the one above, are limited to his understanding of law in the jurisdiction in which he practices and not to any other jurisdiction. In addition, no response to any posted inquiry should be deemed to constitute legal advice, nor to constitute the existence of an attorney/client or other contractual or fiduciary relationship, inasmuch as rendering legal advice involves the ability of the attorney to ask appropriate questions of the person seeking such advice and to thus gather appropriate information. In addition, an attorney/client relationship is formed only by specific agreement. The purpose of this answer is to provide the questioner with general information, not to outline specific legal rights and remedies.