What is meant by 'two headed Janus'? Why is he referred to in the extract?

Not in love neither? Then let us say, you are sad,
Because you are not merry: and 'twere as easy(50)
For you to laugh, and leap, and say you are merry,
Because you are not sad. Now, by two-headed Janus,
Nature hath fram'd strange fellows in her time:
Some that will evermore peep through their eyes,
And laugh, like parrots, at a bag-piper;(55)
And other of such vinegar aspect,
That they'll not show their teeth in way of smile,
Though Nestor swear the jest be laughable.