When i was skijouring. I used only 1 dog.
Pretty straight forward. Body harness for the dog. Single reign back to the skier with a quick connect/ disconnect joint that ataches to a human waist harness. Similar to a wight lifters belt.

Myself, ive never witnessed more than 2 dogs in a skijor team. The reign from human to dog is about 8'. With 2 dogs. Its not anything different. A pair of 8' tethers connecting to separate connect/ disconnect on your belt/ harness.

With one dig. Hook up & go.
With 2 dogs. Some training is required to keep the dogs working in unison. But, its not rocket science. The dogs figure it out pretty quick.

For dog sleds. Those reigns/ tethers are different. More restrictive for the dogs. They are tethered side by side as well as front & back. The dogs really have no choice but to run behind the lead dog. With skijoring a tandem. The dogs have freedom to run side by side, or split apart a foot or 3. Either way. The dogs pick it up pretty quick. They love the idea of running. Instinctually. And the guy behind them shushing theough the snow incentivizes the dogs to run straight.

You start the training process. 1 dog at a time. 1 session. An hour or 2. The dog has the concept figured out. Once both dogs are competent solo. Then you pair them.

The dog i had. A husky mix. He picked it up 1st day. From that point on. When i put that harness on him. He was so excited to get going. Lol. Keeping him still while i geared up was the bigger challenge. Handled by a stake in the snow out on the public trails. At the high school track. The had a couple of poles in the ground to harness your dog up to.

That skiing accident sounds heinous! Ouch!