I finally got it together to put two dogs on the Tern D8 folding bike. The milk crates were acquired from Home Depot (I am well past my "liberating it for the revolution" years), and after experimenting with various methods, I went back to the one that was first advised: zip ties. They are very cheap and fast to put on, and though they will have to be cut off the back rack every time, it's a total 10-cent cost.
The front crate is zip-tied to a Kanga rack that clips into a Tern luggage truss. The rack and truss are ridiculously priced, but they do the job. and due to the configuration of the Tern, it seemed to be the only thing that would do it proper. The good news is that the rack and crate come off as one, and with a simple push of a button. Perfect!
Sunday was our first roll of any kind, and we did 20 miles on the C&O Canal in 95 degree heat without a single glitch, other than I shot past Pennyfield Lock on the way back, which cost me a total of 5 extra miles coming and going. This was easy biking!
The dogs are attached to the inside of the basket with a very short piece of leash -- so short they cannot leap out, but just long enough that they can sit up and hang their head into the breeze.
Next time, I'm taking a real camera and a fly rod.