Rainbarrel Example:

This homeowner tied two rainbarrels together to form a tight fitting unit:

 

Note the 2 inch piping that is used to tie to the gutter system. Note the stocking used between pieces that serves as a "filter". The pipe is threaded into a bung hole:

The overflow is threaded into two bung holes, and tied between the two barrels. A hose at the left carries overflow water where it is wanted:

The spigot system is tied to both barrels: