ok well here is what I would suggest, get a sterilite shoe box from walmart, a small ceramic water dish(cat food dish or plastic one will work too) also from walmaft. This is cheap and and a good alternative to buying another tank set up. Then you need to get some flexwat heat tape, check out
www.thebeanfarm.com they have it again not very expensive, get a table top lamp dimmer, walmart or hardware store approx $10. By an infrared temp gun. By the time you get every thing you need you will not have spent much.
Line the bottom of the tub with paper towel so you can monitor for regurgitation or to monitor the poops. Make sure to drill air holes in the tub and that the tub you get has a secure locking lid(that's why I suggested sterelite). Set up the heat tape plug it into the lamp dimmer, plug lamp dimmer into the wall. slide the dimmer so that it is on about 1/4, wait about 1.5 hours temp check with gun. Remember only 1/3 of your container should be sitting on the heat tape.
You want the temp to be approx 85 on the warm side. Once you can get the temp stabilized place the snow in this set up. It is likely that it is feeling stress living with the other snake. I am not going to jump down your throat for cohabbing, I personally do not agree with it for all the risks it posses and because I lost two snakes as a result of it, so it is not a chance I will ever take again.
What you are seeing in the poop can be a few things, partially undigested food matter, parasites or normal "hatchling" type poop. If the snow is very small just around hatchling size they often have a watery stringy type poop, not very well formed, but seeing as how you have already seen signs of stress I would move it out.
Then once you have it on it's own watch it's behaviour, it's feeding and pooping if things improve then likely it was stressed. Cohabbing can stress snakes enough to reduce their immune system and make it hard for them to digest or fight of illness, as well it can give natural bacteria(the bad ones they carry) or parasites a chance to flourish and overload the gut.
Hope this helps.