I'm with Dena on the worm issue. When I read your description of their bellies it's what occured to me, as most "strays" are born with them.
Dena, if they aren't eating solids yet and are still being dropper fed formula - would their poop be formed? Or more like toothpaste? (not meaning to get too gross here with poop descriptions)
Sometimes without any "bulk" to the stool there just isn't enough pressure on the rectum to stimulate evacuation.
As to the remedy? I'm no help at all
I may be totally wrong here but I'd think they should at least be passing urine more than once since you've had them? It's obvious they are getting enough liquid intake.... I don't know... just something that came to mind.
What Dena said, again, makes a lot of sense. They could be just fat 'n happy and their bodies are absorbing all that nutrition they were lacking for who knows how long.
I wish I could be of some help! This must be stressful for you.
I'll keep you guys in my thoughts! Hang in there Becki , it sounds like you are working so hard and doing everything in your power to give these little guys a great start.