Two bodies have I, though both joined in one. The more still I stand, the quicker I run. What am I?

“A clock”

While a clock tracks time, it does not have two bodies that are joined in one, nor does it have the characteristic of standing still to run quicker.

“A candle”

A candle does burn down over time and can be seen as a measure of time, but it does not consist of two distinct bodies and does not fit the phrasing of standing still to run quicker.

“A pair of shoes”

A pair of shoes does consist of two bodies, but they do not measure time or have the specific qualities described in the riddle.