My first thought would be spider bite or something like that but two dogs with swelling in the same exact area of the neck seems to coincidental.
A quick google shows this:
Among other things, this can indicate an infection somewhere in the body as well as a condition called juvenile cellulitis which, if left untreated, can eventually cut off the puppy's ability to breath. It can also indicate the presence of a tumor, benign or otherwise. The sooner you can have it determined which of the three( if any) it is , the sooner it can be treated.
If the swelling is going down, I'd be thinking insect bite, especially if the pup is eating normally and playing normally. If the pup goes off his feed or gets lethargic, I'd take it (them) to a vet.