Q. In our times, do legal documents need 2 kosher witnesses (or are is the signature of the parties involved enough)?
for example – when one writes a kosher will.
is his signature enough, or does it need 2 kosher witnesses.

A. Responsa Tzemach Tzedek (10) in dealing with the validity of cheques in Halacha, rules that one’s signature carries all the force of witnesses, and obliges equally the signer, see also Rashbo (Tesh. 3: 67, 4: 7). However, one’s signature on a loan or any promissory obligation or payment without witnesses does not create a lien on real properties (real estate) and cannot be used to collect from properties that were sold subsequently.
Bach Responsa (15) rules similarly and compares it to a document common in their times know as “memarani,” that only carried the signature of the borrower or payer, Igrois Moishe (C.M. 2: 15 in regards to sh’mitas k’safim writes that since a cheque has validity as payment by the law of the land, although it does not have any witnesses, only one’s signature it has the same effect. (See also Minchas Yitzchok 5: 120, et. al.).
Horav Shlomo Miller’s Shlit’a opinion in regards to wills is that in principle the testator’s signature is binding without the need of witnesses, however, there could be other complicated issues involved besides collecting from encumbered properties mentioned, and cases differ. Therefore a competent Rabbi should be consulted before writing a will.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a