La prueba testifical ha sido enormemente ilustrativa y esclarecedora para conformar la íntima y certera convicción de culpabilidad de ambos acusados por parte de este Tribunal.
The witness testimony has been enormously illustrative for this Court to arrive at a clear and convincing belief as to the guilt of both defendants.

Black’s Law Dictionary (9th ed.)
clear and convincing evidence. Evidence indicating that the thing to be proved is highly probable or reasonably certain. This is a greater burden than preponderance of the evidence, the standard applied in most civil trials, but less than evidence beyond a reasonable doubt, the norm for criminal trials. Also termed clear and convincing proof.

--------------------------------------------------
Note added at 12 hrs (2013-06-28 00:29:54 GMT)
--------------------------------------------------

I don't know where my head was at earlier. Should’ve read my own Black’s cite. For criminal trials it is, indeed, “beyond a reasonable doubt.” So here is how I would re-word the TL sentence:
The witness testimony has been enormously illustrative in clearing things up for this Court to arrive at an opinion, beyond a reasonable doubt, as to the guilt of both defendants.

One thing I would caution is to stay away from the convicción>conviction translation in this context because the English word is used in the legal arena to mean a finding of guilty as a matter of law, not as a matter of fact.