It comes from contaminated areas. Yards, ponds, puddles, lakes, streams, etc. It gets contaminated from feces of wild animals or other pets. If they're both in the same infected area, they can both get it. Or if they're poop eaters........
Ours have never had it. so I can't help much, but one thing you need to do is to make sure you also double check if there have been more incidents (than normal) of Lepto in the area. That also comes from contaminated water and if it's been very wet or rainy in your area, the chances of runoff into normally clean water supplies is much higher.
If they're seeing more Lepto...make sure your dogs are UTD on their vaccination for it. If not...that's good.
I had a friend with giardia who had no clue where she got it. Never went camping or swimming in any kind of water that "should be" contaminated.
It took her a couple of weeks on Flagyl (which is metronidazole) to get rid of it. The Flagyl was hard on her .. she said it was worse than the giardia.
I notice in the other thread you mention yours are on Panacur (which is the most acceptable treatment for giardia), but perhaps you might want to ask your Vet about the Flagyl/metro if you aren't seeing an improvement?
Don't know how you decontaminate, but here's what I found:
The most readily available effective disinfectant is probably bleach diluted 1:32 in water, which in one study required less than one minute of contact to kill Giardia cysts. Organic matter such as dirt or stool is protective to the cyst, so on a concrete surface basic cleaning should be effected prior to disinfection. Animals should be thoroughly bathed before being reintroduced into a “clean” area. A properly chlorinated swimming pool should not be able to become contaminated. As for areas with lawn or plants, decontamination will not be possible without killing the plants and allowing the area to dry out in direct sunlight.
http://www.veterinarypartner.com/Content.plx?P=A&A=2386