Tandem rosettes are what you call it when the ring is built into the bridle rosettes, also normally called the conchos. "Roger rings" are what you call it when the rings are suspended from a strap, either from the throatlatch or sometimes from the crownpiece or other arrangement. Roger rings are the more recent invention and are nice because they keep the wheeler's head from getting pulled around when a sharp turn is made and allow a straighter line from the leader's mouth to the driver's hands. There's a little more "play" in them. When using tandem rosettes it was traditional that the wheeler should wear a snug sidecheck to keep his head steady as any tossing would yank both the leader's mouth and the driver's hands. A couple of my older historical texts talk about that and how it was a great improvement when Roger rings were invented as it alleviated that concern.
I had a set of round rings suspended from short leather straps that were meant to be neck terrets on another piece of harness, so I simply buckled them through the join of the throatlatch, browband and cheek piece and had instant Roger rings. The leader's lines are a pair of ground-driving lines I had around (made myself, and they're not great material as it turns out) and while the length was good I wouldn't do that again. Thin round cord like that turns out to be very hard to keep a grip on when you're also holding wide flat synthetic reins! I extended the leader traces with baling twine and used my regular kicking strap as a trace carrier on the leader. She's also wearing my set of sliding side reins as when we started out that day she got very excited and hollow and thus was not listening to commands particularly well. The side reins got her working through her back a bit better which brought her into contact, something which is very important in your leader. For connecting the traces to the wheeler's breastcollar I followed Breanna Sheahan's example and used a set of stainless steel load-rated carabiners to connect a set of quick release snap shackles to the Freedom Collar. They go on and off very easily and best of all, if (when) your wheeler gets a leg over a trace you don't have to cut the twine but can simply pull the pin on the shackle, release the twine then put it right back on correctly and continue driving. It works great!
You can pretty much do an entire tandem harness out of baling twine if you want to. A couple of small loops of twine at the crownpiece makes Roger rings, more twine can make a trace carrier and extend your traces, the only thing you really have to buy or make for test driving is a set of leader reins and a suitable whip. I use an old lunge whip I have which is missing the bottom half of the stock! That makes the stock about the same length as my regular driving whip but the lash is long enough to reach my leader and it's surprising light as well as being conveniently black. I've been saving it for years just for this! LOL. I've heard that some people use a set of full-size horse reins for the leader, others use long-lines as I did, some find it better to simply buy leather tandem lines.
I gotta tell you...
TANDEM IS A BLAST!! I finally put mine together this weekend using the same setup as in KY with the exception of having proper leader lines this time and can't wait to share the pictures! Kody and Turbo ROCKED!!!
Leia