For reading you should Google Driving Pairs. There is a website with much information on various Multiple hitches there. Hardy Zanke has his articles on progressive Pair driving, teaching the horses, harnessing ideas and skills for the Driver to learn and improve their rein handling. It used to get a lot more traffic with excellent, longer discussions than currently is seen.
They opened a FB group, Driving Pairs, and questions seem to all have migrated over there. I don’t care for ot as well, only snippets of information is shared, not the great discussion details of the first group. It does share many pictures to get details cleared up, which is helpful.
The magazines all have advertising for Trainers. Perhaps you can locate one in there. Not sure of your location, so can’t suggest anyone. Are you in a local driving club? Local being someplace within 100 miles or so! Ha ha The Clubs often have Trainers come in to give lessons to members. We attended a lot of lesson sessions as we improved our skills. No really local Pairs or Fours Trainers near us.
Yonnies does good harness!! Congratulations! We use his shaped breatcollars, they fit well and have kept the horses comfortable. Plus easy cleanup, alway a good thing!