(a) General rule.--In any civil action or proceeding before any tribunal of this Commonwealth, or conducted by virtue of its order or direction, although a party to the thing or contract in action may be dead or may have been adjudged a lunatic, and his right thereto or therein may have passed, either by his own act or by the act of the law, to a party on record who represents his interest in the subject in controversy, nevertheless any surviving or remaining party to such thing or contract or any other person whose interest is adverse to the said right of such deceased or lunatic party, shall be a competent witness to any relevant matter, although it may have occurred before the death of said party or the adjudication of his lunacy, if and only if such relevant matter occurred between himself and another person who may be living at the time of the trial and may be competent to testify, and who does so testify upon the trial against such surviving or remaining party or against the person whose interest may be thus adverse, or if such relevant matter occurred in the presence or hearing of such other living or competent person.

(b) Testimony by deposition.--The testimony now made competent by subsection (a) may also be taken by commission or deposition in accordance with law, and, in that event, the deposition thus taken shall be competent evidence at the trial or hearing, although the person with whom or in whose presence or hearing such relevant matter occurred, may die or become incompetent after the taking of such deposition.