I have 3 that are side by side. I connect them with a pipe connector. It has NPT threading on the inside of the barrel along with a large brass nut to keep it from falling out & garden hose type threading on the other end. Then I use regular garden hose (cut to the size I want)to link them togather. When 1 fills up it overflows into the adjoining barrel.
How do you get the water to the barrels? I use a device called the Garden Water Saver or something like that. I love it! I have to put my barrels about 10 feet from the rain gutter & this lets me do it easily. Plus, unless you pointed it out to someone, they will never even notice it is on your house. Good for fussy neighbors!