Considering there is SO MUCH variance in the costs of a single harness, there is the same in pairs harness... Usually, it's about double the cost of a single - simply because you are using two complete harness's w/ extra pieces. Pairs harness is also made differently - if not using collars/hames (either farm style or buggy/carriage style) - the breast collar has to be "beefed up" in materials and balance because it now carries the weight of the tongue and needs more parts as well. Some other parts of the harness will need to be strengthened & some parts will be different. The "brakes" are different from a single to a pair in a wheeled vehicle - so the harness is different as well.
I started training our pairs with hand made harness braided from recycled haystring (long, long strands from around the round bales we get) - in a single, pleasure style. The first set of lines was a long clothesline cord that was meant for only one. We attached each end to the out side of the bit of each horse (left side to left side of left horse, right side to right side of right horse), then connected them together via another short line from inside ring to inside ring. The breast collars didn't need to be beefed up because they were never attached to a yoke in the "training harness". However, they pulled a "surf board" as a pair, a chain link drag @ my place and then a draft horse drag w/ a seat on it. It would be a few months later (spent over a year - first training them to drive single and then to work as a pair) before we would get "work type" collar/hame harness and then to hook them to the wagon I got BEFORE training them as a pair, LOL...
The purple/white lead line keeps their rumps together, so they don't turn and face each other. This pic is June 2010 - their first time ground driving together.
This pic is October 2010.
2nd hitch to wagon in new harness - March 2011.
Pulling a disk normally pulled by a full size draft horse - April 2011.
October 2012 - pulling the new wagon at a show in TX
As far as tandem, I haven't tried that yet. I have driven our girls single, pair, 3 abreast - hooked to various equipment and wagons & ground driven them as 4 abreast. I have the lines now to hook up a 4-up, but haven't a clue when I will try that right now... This month marks almost a full year since ours were in working condition and actually being worked.