Purchase 1m beer line with a gas disconnect (grey) on one end and a liquid disconnect (black) on the other to daisy chain multiple kegs together. Use this to chain 2 kegs together with the tap pn one and the gas source on the other. For example, as shown below link a 4L keg to a 19L and you have keg volume for an entire 23L brew. Plus the 4L keg is always full and ready to take out with you.
A grey-to-grey is used to equalise the pressure between two kegs or a keg and fermenter before transferring from one to the other.
A black-to-black is used for transferring from a full keg to an empty keg via the liquid posts.