The worst thing about these is that any flat and you are stuck until you fix it. The ones I have seen have a pivot on each set of wheels that equalize them inside to out. If you have a flat the axle pivots the flat one to the ground. Plus the inside wheels are a real pain to change, really bad when loaded.
You should be able to find conventional dauled tandems worth the money right now. I have seen several here in Iowa for the low 4k range. Trail King is a common brand.