what is meant by the 'two headed Janus'? why is he referred to in the extract??

The "two-headed Janus" implies he might just as well describe himself as happy.... it's the ability to communicate why you're sad, rather than being unable to put it into words or know the reason. Janus was the Roman god of beginnings and transitions, his image (two heads) was meant to remind people that human beings often have no center for their thoughts, and in turn, are incapable of knowing or understanding themselves.

The reference to Janus comes in Act I, Scene 1—Antonio is complaining because he is sad, but does not know why, and Salanio and Salarino are making suggestions as to the possible causes of his sadness. Antonio has already rejected several of these—he is not missing the open sea, he is not worried about his merchandise, he is not in love—for all intents and purposes his sadness is inexplicable. Salarino states, after his and Salanio’s suggestions have all been overturned: “Then let us say you are sad / Because you are not merry.” It is not therefore sadness that Antonio feels, but the absence of happiness, and it would be just as easy for Antonio to say he was happy, rather than sad. Salarino goes on to say:

…Now, by two-headed Janus,
Nature hath framed strange fellows in her time:
Some that will evermore peep through their eyes
And laugh like parrots at a bag-piper,
And others of such vinegar aspect
That they’ll not show their teeth in way of smile,
Though Nestor swear the jest be laughable.

Salarino doesn’t exactly call Antonio Janus, but he does speak in reference to Janus when describing his friend’s affliction. Janus was the Roman god of beginnings and endings, transitions, and gateways. Because he was the god of transitions and a warden at the gate, he was also involved in exchanges, and peripherally in trade and travel. He therefore makes an appropriate god for these merchants to invoke. According to the mythology, Janus had two heads—one to look to the past, and one to look to the future; one head to guard a doorway in each direction. This dichotomy is what Salarino is touching upon in this passage: there are men with happy dispositions—one head—and men with sad, serious dispositions—the other head. Salarino is saying here that Antonio falls into the second category, and is sad because it is his nature.