Green’s Dictionary of Slang

1. sexual intercourse involving two men and one woman.

1963		C. Wood ‘Prisoner and Escort’ in Cockade (1965) I iii: I’m thinking of chatting up the princess – I promise you ... two’s up.
1998		K. Sampson Awaydays 5: A highly involved and highly unlikely-sounding tale casting himself and Ally Quinn as tireless Lotharios in a twos-up with Toothless Elsa, one of The Tub’s glamorous barmaids.

2. spec. when a woman has two men simultaneously penetrating her anus and her vagina.

2001

‘Reviews’ at Xesshop.com Catalogue 🌐 ‘Dangerous Tides’ [...] These swingers are really up for it and we especially like the avid back-door action. A horny two’s up and Jenna Jameson having a great time makes this a great film. [...] ‘The Kiss’ [...] Some very sexy action, a bulging two’s-up and in true Jenna style, lots of oral and cum shots.