All you need is hollow rope and a fid (the black thing in the above pic). Lehigh company supplies one with each bag of rope.
Simply thread the end of the rope into the fid. You need to have at least 6 extra inches of rope for the the splice. Put the end of the rope into the fid and thread it through the rope and pull out the other side.
Then, thread the fid through your loop (which has been pulled down to the size you want it to be).
By the spot where you originally threaded the fid through the rope, put the fid down the center of the hollow rope.
Feed the fid & rope through the center of the hollow rope until all the extra rope had been pulled through. Snake the fid out and you've done a loop.
So, with this new piece of information, I built myself a bikejor line for two dogs. I still have to buy the bungee to insert but other than that, it's done. I got all the parts at Lowe's for $18. (That's the rope, 2 steel rings & 2 swiveling hooks.) I made sure each piece was rated for more weight than my dogs are. The various parts are rated for 170 - 300 lbs working load.
I'm not sure how long it is but it's longer than what we've been using so I'm happy about that. I've been nervous about running into the backside of one of the dogs.