Packing 2 planes in a standard 20' container is tricky business. It is not the feet cubed of volume that are sparse, but rather the square feet of side surface. The two wing halves of a SEP will each cover one side, and where do you put the other two?
If it must be done, I should think of a rough timber of jigs, a lower one to store two fuselages and an upper one for four wing halves. Make to fit almost precisely, then wedge them, and immobilise the wedges.
FWIW I was once incurred in the shipping of a Weed-Hopper type ultralight, the packers declared themselves unable to pack the poor little thing into a 20' container so they persuaded the buyer to pay for a 40' one - all the way from Antwerp to Thailand. But they were obviously clumsy at handling such a delicate load, and apparently didn't like my telling them so...