I think it's just a matter of training the pups - and I think it's important to train them one-on-one. Like I would teach Ollie how to walk on a leash w/o Tia there. It takes practice and patience.
I only have Jackson but have walked up to 4 dogs at 1 time and it's a whole heck of a lot easier when they are all trained... my dad has 2 dogs so occasionally I walk all 3 together and it's difficult because one of them is a huuugeee puller, causing the other 2 to wanna pull too, LOL. Yet when I walked my mom's friends 2 dogs and Jackson all 3 together, it wasn't too difficult because none of them pulled. It's always the hardest at first trying to get all the leashes organized. lol.