I after a long search in Aus, did what another fellow had done, and that is use a small Aunger box trailer, but I was not bothered about carrying two AIs. I was much more focussed on saving money too! I am sure that two could be achieved, by stowing the hulls upright, father than flat. A long thread on an Aussie kayak site covered my issues, but I am very happy with the result, other than noting its far easier to load and unload if the AI is much closer to the ground, which precludes using a box trailer:
Road Trailer for a yak
Mine has a torsion bar suspension, with shock absorbers. A typical axle springs and wheels setup will start at $600 before you add any steel to your trailer.
If I had really wanted to carry two AIs, I suspect I would have not sold the H14 trailer I had, that went in excellent condition with rego for $370. That makes a wonderful starting frame. And its low to the ground.
Its not essential to have 13 inch wheels. I can tow at 110 km for long distances with 10 inch wheels. Not sure where you are seeing spring packs at 600kg, I have never seen a cat trailer with more than maybe 200kgs all up. Or are you referring to the load its expected to carry? I guess stuff designed for stinkers will be heavily rated.
This thread http://www.hobiecat.com/community/posti ... st&p=61570
is also with an Aunger trailer, same as mine, but with the sides taken off. Its a good option.