First off I advice that you get a mentor or trainer to help you out. I'm in the process of refining my miniature horse's cart training, but I've been driving for about a year now and have had a trainer help me every step of the way. Green plus green equals blue, and driving in some ways is even riskier than riding so you must do things right! They can definitely pull more than 40lbs...whoever said that was probably referring to how much they can carry
which is much different.
As for how much they can pull, that depends. Do you know their heights/weights? They sound like they're just pasture puffs right now so as you get them in shape, their weight limits will go up a little bit as well.
My trainer has told me that the generally accepted amount that a mini can pull is 2-3 times their weight. Only three times their weight once they are fully conditioned and finished growing (4-5 years old) and only on flat surfaces. Remember though, that you have to take into account the weight of the harnesses, your buggy, and you.
For example, my miniature mare (coming four years old this Friday) is 34" tall and 225 lbs (give or take a little). Her harness weighs 10 lbs (all leather), and my cart weighs 90lbs, (its an easy entry miniature horse buggy) Two times her weight would be a little over 450lbs. This means that she can take up to 350 lbs of passenger weight. I weight ~90 pounds, so anyone riding with me would have to be under 260 pounds...easily a full grown man. So yeah, I'd say two small to average sized adults would be very fair.
You also have to take into account the terrain and duration of the ride though, ofcourse. We have a shetland pony who is about 39" tall and 400 lbs, so he could TECHNICALLY pull 800 lbs, but because we use him for payed rides and he works anywhere from 2-3 hours at a time at a trot, we limit weight to 500 lbs. You just have to use your logic ^^
An idea of what my buggy looks like:
Good luck and have fun! Miniatures are so much fun and are very addicting!