Refers to a girl who you have
on the side.
You have a wife or steady
girlfriend - she is your (#) One.
The girl on the side or the one you stash away - she is your (#) Two.
Commonly used by musicians to describe the girls "on the road."
In some cases, even if you have no "number one," a
random hookup or a girl who will never be considered for the "one" spot, can also be considered a "two."
Also referred to as "doing the two."