For kid's trailers, I prefer trailers with a wrap-around frame, and using standard quick release wheels (usually dished outward). Unfortunately, many of the new trailers are moving to the single-sided wheels which I believe is an inherently weaker design.
I've found a couple of brands that make (or made) full welded frames, either steel or aluminum. Possibly the same manufacturer with different brand labels.
It should be easy enough to take one of these welded kid's trailers and extend the frame to say 8 feet or so, for two large dog crates. I'd probably take the frame, and bolt something like aluminum ladder rails to it. Then figure out how to re-make the hitch and get everything rigid.
The spring hitches that are popular for easy flex are limited by weight, and become uncomfortable for riding with > 100 or 200 lbs cargo. Still, they will work if you can keep an even cadence and circular pedal stroke.
I have seen a few people towing two kid's trailers. Probably not my choice, but it has been done.