The real answer is none except they don't leave a ridge in the middle.  Most offsets are heavy, but a tandem could be built as heavily as an offset and would cut as well.  Typically an offset is only one section, although there are some folding ones made.  When it comes to 36 and 42 inch blades there is too much weight for  hinges, so they are almost always a single frame.  The typical large offset has five or six inch lift cylinders, and it is a lot faster to simply line out the field to make all left turns than to wait at the end of each run for the lift cycle.  One other advantage is that it's a little easier to change blades and bearings.  I've banged the back of my head too many times on the middle sections of a tandem.