Green’s Dictionary of Slang

1. (UK prison) having two prisoners sharing a cell; thus threed-up etc.

1977		G.F. Newman Villain’s Tale 136: The cells were more crowded, everyone was threed up, some even four to a cell.
1992		D. Burke Street Talk 2 142: Two’d up – number of prisoners in a cell. Can also be ‘three’d up.’.

2. in non-custodial use of sense 1.

2011

J.J. Connolly Viva La Madness 22: I would really like to be two-ed up with Morty so I could ask him what the fuck is going on.