It hangs like a curb chain (note: the judges at this World Show had no clue what it was and even asked my wife, the groom, what it was).You also see it on four in hands and often on pairs. Get a rein caught on the adjacent horses bit and you have a real problem. The closer the better on pace. The lead must be absolutely solid and the wheeler must be happy to do all the work with the view never changing. . You may have to try them both ways and see which works best for them.
Another tip. Get a set of rein clamps. You can see in my left hand that the reins loop thru my hand. In this picture the clamp is actually in my hand. If the lead horse is too far out simply walk up the lead rein. This will pull him back without affecting the wheeler. If he is too close simply walk down the rein and he can move out. Without the reins being clamped this is a difficult process. The overall length of the reins never changes. You are simply adjusting their position. This also is a must for four in hands. Make sense??
Once you get the hang of it, it will be more fun to drive than anything.