I have not had coffee yet but here are my thoughts.
Sounds like the pair is getting a long fine despite not doing a traditional introduction. I have done a pairing with no intro, no scent swapping, and it was successful. It's not what I usually advise a newer owner though. Which is here nor there at this point in this case, it's already done and sounds successful.
My concern is the runny poo. Diarrhea is a symptom of something else, most often something with the diet but that is not the only cause.
I know you said your feeding bml, but what is the exact recipe your using?
One of the most common mistakes with diets is modifying the recipe, whether you modified it or someone else did. Please post the recipe you are following so I can see if it's correct. Don't leave anything out. What fruits n veg are you feeding, how are you preparing them and serving them ?
The cage does sound like a bird cage which maybe to small for 2 gliders. However, I don't like to assume anything, if you can post a pic of the cage that would be more accurate. I do not judge, but will make suggestions if need be, if you rather pm me pics that is fine too.
The sugar glider usa store... looks like a distributor for Exotic Nutrition. The only thing I'd buy from Exotic Nutrition is the cages. The food, snacks, toys, pouches wheels are pretty much crap.
I did browse the store, really there is only one thing I would buy from the 3 pages of products I looked at and that is the bottle nurser with miracle nipples. Those are good to have even if you aren't a breeder, they are good for emergencies and rehydrating gliders or hand feeding if the glider is ill.
There are a lot of products on there that have issues or safety concerns for me. The Kaytee cleaner deodorizer spray for instance, an owner used this and believed it to be safe, on the website it says you can even spray it directly on the animal. Well she did and both her gliders died, this was 2 days ago.
I know your trying to do your best, and provide what your gliders need. I also know that these products seem like good options but most of them are not.
The wheel you ordered is probably the silent runner. While it is suppose to be an upgrade from the wodent wheel it still has some kinks that need worked out to be deemed a safe wheel by the community. One of the major issues with this wheel is it may not come assembled. This also means you can take it apart and put it back together. This would seem like a great idea right? well it's not. This means human error can happen and will more often with this wheel. Which means gliders are going to get injured.
For Glider products the best ones aren't commercially made. They just aren't. It would be nice if they were but unfortunately gliders are thought to be enough like rodents by the commercial companies, that rodent and other animal necessities like dust baths for chinchilla's are offered for gliders. This poses some issues, especially for new owners thinking they are buying the right things.
Do you have a glider vet? This would be my first suggestion. Get the gliders in for fecal tests and I would suggest not just one. Some parasites don't show up in the first fecal test. So a 2nd fecal test is needed 30 days after the first one. Tell the vet you want the one with runny poo tested for giardia and trich.
Both giardia and trich are highly contagious parasites and if one glider has parasites ( no matter what kind ) they will both need medicated.