Making the beast with two backs is a euphemistic metaphor for two persons engaged in sexual intercourse.
It refers to the situation in which a couple—in the missionary position, on their sides, kneeling, or standing—cling to each other as if a single creature, with their backs to the outside.
In English, the expression dates back to at least William Shakespeare's Othello (Act 1, Scene 1, ll.
Thomas Urquhart translated Gargantua and Pantagruel into English, which was published posthumously around 1693.
[3] In the vigour of his age he married Gargamelle, daughter to the King of the Parpaillons, a jolly pug, and well-mouthed wench.