There is a difference between a tandem and a double offset.  On a true tandem the front and rear gangs are directly across from each other.  On a double offset, the left side will be slightly staggered from the right side, so that the front gangs overlap slightly, eliminating the middle ridge.  That said, I made most of my living the last several years running offset discs; mostly Towner and Rome, and I don't think there is any question that an offset is more trouble free than a tandem or double offset.