Sorry for the loss of your rabbits. The fact that both rabbits were affected tells me that the culprit was infectious disease. Dehydration and shallow breathing means that the disease was primarily of the respiratory system. The most common infectious respiratory disease that could be this deadly is pasteurella. Pasteurella is a bacteria that infiltrates the upper and lower respiratory systems of rabbits and other small mammals. Death occurs by complications that arise from pneumonia.
If these rabbits were house with, or had recent contact with any other rabbits, you should get them to a vet ASAP. Pasteurella is treatable when caught very early, but it is often beyond treatment once the rabbits begin to show clinical signs.
Again, I am very sorry for your loss.
Best of luck to you, and thank you for choosing JustAnswer! Please do not hesitate to let me know if I may be of any further assistance.
Sincerely,
Roger L. Welton, DVM
President, Maybeck Animal Hospital
Author, "Canine And Feline 101"