1. At least two of the subscribing witnesses to a written will shall be examined if they are alive and competent to testify and otherwise available. Before any will is probated each of at least two witnesses thereto shall testify to facts showing that the will was executed in accordance with section 474.320. This section does not alter the rules of evidence as to the establishment of a will by probate in solemn form or affect the probate of a self-proved will.

2. If either or both of the subscribing witnesses to the will are dead, physically or mentally incapable of testifying, or their whereabouts unknown, then due execution of the will by testator and its attestation by subscribing witnesses shall be proved by the available subscribing witness, if any, and proof of the handwriting of any dead or mentally incapacitated subscribing witness or subscribing witness whose whereabouts is unknown, or by such other competent evidence as is available.