I think you might want to read up on the Achenbach system of driving to begin with, which uses reins that adjust uniquely for Pairs. Max Pape wrote “The Art of Driving” which is about what he learned from Benno von Achenback, a renowned master of Driving skills. A secondary book would be “Driving, The Complete Riding and Driving System: Book 5”. It is the official instruction handbook of the German National Equestrian Federation, and follows their methods of training horses. It includes information further expanding the Achenbach System of driving horses.
Both books are well illustrated to show what is being spoken about in the texts. The special rein system is also well illustrated and explained.
Now back to the original question about splinter bars, roller bolts, and eveners.
When we start a Pair, the evener is tied down, so horses are only pulling with the singletrees. NO CHANCE of Eager slamming Slow with the collar pull of the evener. If Slow doesn’t start at the same step, he has to catch up, but is not punished by the equipment, to make him WORSE about starting. It can take TIME, persistant Driver effort, to get Slow stepping off with Eager at the “Walk On” command. You may want to work Slow alone, get him stepping off quicker, promptly, when he hears the command. He MUST do his part eventually, to quit frustrating Eager!
Working the horses alone, you can soothe Eager, try to make him HAPPY during his session. Then touching up Slow for a crisp response if he still ignores you during his session. Use their names, same crisp voice, but let Eager stroll off to start, and Slow may get a touch of whip if he persists in ignoring the command. Horse name, SHOULD mean something to him, get his attention, THEN give command, for prompt response. This can take a while to get smooth in the Pair, ANY PAIR learning to work together.
We drive with fixed straight poles or sprung poles like Marathon vehicles have. No real downward pull with yoke or tongue weight. Marathon vehicle has a metal yoke on a short pole, can’t fall off, rotates up and down, forward and back with the horses. Pleasure carriages we own have singletrees, with the Marathon vehicle carrying an evener with singletrees. Having singletrees on everything is because we usually drive in breastcollars. The Pleasure Vehicles also have splinter bars with roller bolts, if we want to use them we remove the singletrees which are hung from the splinter bar.
Collars are used with splinter bars, never breastcollars. Collars have more surface on the horse for pulling, better movement with the shoulder of the animal. I am going to say that while splinter bars are traditional, we don’t use them much because it takes time and effort to keep horse shoulders fit for work wearing a collar. It is VERY easy to sore or scald a horse under his collar when working them. You probably know about keeping shoulders fit for collar work already!
I don’t recommend using a splinter bar setup, for your people mover. Fixed poles that go with splinter bars were never designed for that kind of load. Heavy Coaches that did haul loads of people on their routes, went thru a lot of horses because they got sore with the weight. Fixed poles do not use a yoke, the horses are fastened to the pole end with chains or straps from the collar to the pole end. Pole is LONG, needs to be a couple feet longer than the horse nose when he is relaxed while hitched. The length gives horse Pair leverage to control their vehicle behind, and straightens the pull from forward pole end, so they are not being pulled sideways. Horses are hitched in slightly snug, so there is a bit of tension from front of pole all the way back to the splinter bar and roller bolts that traces attach to. Horse can move “within” the harness, but harness is NEVER hitched loosely for the fixed pole, splinter bar type of setup. Horses get slapped with harness and weight for stopping, starting, if there is no tension from front to back in the hitching.
So my suggestion is to try locating either or both of those books, learn about the Achenbach rein system for Pairs. Figure out what measurements are needed for your size horses and order a set of Achenbach reins. Leather or synthetic, correct size for your hands, so they fit. While waiting for your reins you can do single work with both horses, to get them understanding what you want with the commands. Then you can adjust away with a set of Achenbach reins while driving the Pair and be surprised that they actually work!
I think you have an equipment issues with the reins used now. Horses needs some work single, to let them be better and more responsive to direction. Tie down the evener when you put them together, that see-saw start is REALLY HARD to work with for both animals. So get rid of it by tying the evener down to let them pull off the singletrees for a while, they don’t punish each other. EVERY set of 2 horses has an Eager and Slow. You can put 2 Slows together and one will turn into Eager! Same with 2 Eagers, one will turn into a Slow. It is amazing to us, but it happens with every Pair we know, no matter how you switch them around.
I wouldn’t bother with changing my equipment on the people mover at this point, beyond tying down the evener. I don’t think the splinter bar, roller bolts are the answer to the problem.