It is in Chapter 3 of Book II, entitled "A Disappointment," that we are introduced to Darnay's trial for treason against the English crown and we also meet the two hostile witnesses that testify against him. The first of these hostile witnesses is John Barsad, who later on in the novel turns out to be Miss Pross's long lost brother, Solomon Pross. Note how Dickens introduces an element of doubt into Barsad's account:
The story of his pure soul was exactly what Mr. Attorney General had described it to be - perhaps, if it had a fault, a little too exactly.
Note the irony in the description of him as a "pure soul" which is shown by the immediate suspicion that he is making up his accusations because he has repeated them "too exactly."
This is a technique that is used to present the other hostile witness, Robert Cly, "the virtuous servant." Also note the element of humour that is inserted as Cly tries to prove the "virtuous" nature of his character:
He had never been suspected of stealing a silver tea-pot; he had been maligned respecting a mustard-pot, but it turned out to be only a plated one.
The ridiculous lengths that he goes to to try and establish his good character clearly hints at deception somewhere along the path in his evidence.
So, the two characters who are hostile to Darnay and testify against him at his trial in England are Robert Cly and John Barsad. It is important to remember that Darnay suffers two trials in this novel, and an interesting exercise for you to do would be to compare and contrast them, identifying similarities and differences.