The testimony of unbelievers and disfellowshipped or disassociated ones may also be considered, but it must be weighed carefully.
That's a direct quote from the elders manual.
In other words, testimony can be used but of course the elders will be more likely to think they're lying.
By the way if you want to get your terms correct, the part with the testimonies is not the judicial committee. Before a JC is initiated, the (two) elders will go on a fact finding mission to see if any sins have been committed that warrant a JC. They will talk to witnesses to the sin (including possible unbelievers).
During the JC there is not much fact finding to do other than quizzing the accused to see if they are repentant.
Only in very rare cases (according to the elders book) it may appear during the JC that a JC isn't needed after all.
Example: while his wife is out of town, a bro is seen walking around town with a beautiful blonde woman. They are seen walking with arms locked together, hugging, during a dinner for two, going into the house of the bro together in the evening, and spending the night there.
Three different JW rat on the bro to the elders. It's clear as day that something sinny is going on here, so the bro is 'invited' for a chat at the Kh. When he gets there, he is ambushed by a 3 person panel of JC judges (who never clearly explain that fact nor the procedure they're following).
They explain charges of immorality and infidelity to him and want to proceed with the JC.
The bro laughs, produces a family picture and points out his beautiful blonde sister who lives abroad and is visiting him for a few days. Yeah, end of JC.