Hi Amanda,
I would just stop and get the girls to get back beside or behind you and then start again, and then stop if they pull, repeating this process untill they understand that they won't get anywhere until they stop pulling. I wouldn't take the dogs with you yet if you have to get somewhere quickly with the buggy as you'll simply be reinfocing their pulling by being inconsistent with the stopping and starting, because I'm sure in the end you'll give into the pulling in order to actually get to the place you need to be.
Set aside a walk time that's just for you and each dog individually. At the beginning you may get no further than the end of the road, but you must be consistent about discouraging pulling. Once both dogs are okay on their own and have stopped pulling then you could try walking them together, however if they pull then you need to go back to singular walks and reinforce the non pulling training. Finally once the dogs are walking nicely together, then introduce the buggy.
This process will probably take a while and may well progress in fits and starts, especially if both your dogs are young and excitable. But it is very important that they get some focused one on one training time with you on the pulling problem, as this is the only way they'll cotton onto what you expect of them.
I've found that treats aren't necessary as the progression of the walk provides enough incentive for the dog to stop pulling and start walking properly
Good luck amanda, I'm sure you'll crack it in no time
Best wishes
Mary