I see that I was quoted here as a possible help... Honestly, I'm still learning about the balancing of cart to both single and pair.
In my case, it's more difficult simply because I use a larger forecart (Hafflinger sized) - which is longer, wider, larger tired and 100+ lbs heavier than the smaller pony sized forecart. I do use it with a pony sized tongue. It's too heavy and too long for the smaller ponies (35,36 & 37" @ withers) that I have to use as a pair - the tongue points down and weighs heavily on their necks. I am a large woman, and I can't shift my weight back far enough in the bench seat to get the forecart to tip back with them - not sure that attaching equipment (too heavy??) would help much in this case. I"ve been considering the lighter forecart pictured below - which is about 1/4th the size/weight of the farm one that I have (meant to be used for farm work).
I've noticed that different companies make a lighter style of forecart such as this one -
I did have a scare one time with this bigger forecart. I was at an event and gave a ride to the farm owner from point A to point B. He was also larger. When he got in and shifted back against the seat rest, the tongue flipped up and the whole cart tipped backwards. Boy, was I surprised as were my ponies!! Could have been a wreck - glad I'd done so much work with the girls. I hadn't even thought about that possibility at that time (had already experienced it with a single and cart w/ too loose of a wrap strap - totally freaked out that pony and ME). Glad that they were surprised but behaved - the one mare's filly was also attached to her dam.
I have been told, numerous times, that I'm wrong and very hard on my ponies using a forecart w/o equipment attached due to the balance/weight issues where the cart can tip forward/low and really weight heavily on their necks. I DO spend a lot of time checking my mares when out for extended times - collar fit. I've found several times that I really have to adjust where/how I sit near the end of a drive - especially with the ponies who aren't as fit. As they tire and/or start working with their heads/necks lower, the tongue shifts down and the cart tips more downwards. It can become a vicious circle where the cart/tongue then continually put more weight/drag on their necks.
Even my farm wagon with 4 wheels has a tongue that weighs heavily on their necks. I've been discussing putting something on it (under the tongue, attached to the wagon) to "suspend" the tongue so it's much lighter on their necks (like with a marathon carriage). I have purchased a mini tongue to use when I hook my smaller than 40" ponies to it - haven't had them hitched yet.
I just lost lots of info that I'd typed and I'll try to re-type part of it...
I noticed when I was at Nationals in 2012, that the chariots seemed to be fit to the individual horses - both singles and pairs. there were different tongue placements - seemed to depend on the axle heights, wheels and the actual build of the chariot... I had some association with chariot racing when I was a teen - sadly, the mechanical/balancing part (or driving) wasn't part of that. I could have learned much. But pictures in the Western Horseman show different tongue/axle arrangements for the big horses, too.