Popular amongst capwearing bus-stop dwellers, meaning to leave half of a cigarette for someone else. A chav equivilent of puting dibs on something.
Can also be applied to other areas when sloppy seconds are acceptable, for instance:-
"two's up on your sister, mate."
Can also be used a term for a threesome. For example:-
"Alright love, how about me and my mate go two's up on you at your place?"