have they had their first shots yet? Were they check out by a vet before you purchased them? I would consider taking them to your vet with a stool sample, and having it looked at. Usually bloody stool isn't good.
Did you purchase them or are they your pups that you bred?
Have they had their shots yet? Have they been wormed, stool checked by vet?
My first reaction is coccidiosis, but you could also have a worm issue or even parvo. Get them both in ASAP and take a stool sample from each pup with you. If one has it, it is only a matter of time before the other pup also has symptoms.