This trick is a variation on columns. It looks like you're cheating, but in fact, it is actually more difficult to pull this off.
Start out by doing regular columns, holding the one ball in your non-dominant hand, moving it up and down in time with the other outside ball, without throwing it.
There are many other variations on columns, such as: crossing the two outside columns in the middle, moving the middle column to the outside, throwing the middle column over top of the other two, and countless more, use your imagination.