Shipping Two Cars

I would ship them inside of another padded box. Handling can get pretty rough during shipping. Smaller boxes are more apt to be used as filler boxes during shipping. I would use as large a box as possible, within reason, to ship. Your cars will be much safer.
 
I ship together and wrap the two car boxes in plenty of bubble wrap so they can't move around - never had a problem.
 
I use the free USPS flat rate boxes, either medium or large depending on the number of car boxes I can pack in them. Pad them well so they do not move around inside the flat rate boxes.