Hi Kate,
I've had Giardia and Coccidia in the past. Both are pretty common occurrences since we live near one of the world's biggest fresh water bodies of water. Our wildlife tends to carry it and transmit it via their urine and feces in our soil and drainage water. Doesn't mean that I keep my dogs in kennels, or that my dogs live outside or that I keep a dirty home, rather, it's just the nature of our geography.
The powder that you're using is likely Panacur. I use it occasionally to treat my girls when they've come home with the runs after a swim. I just make sure to keep some on hand and use common sense when allowing them access to the lake or rivers. We don't swim in stagnant water and make sure not to swim after big rains.
For treatment of your yard, try spreading lime. It neutralizes the soil and kills the buggers. Simply place it in a fertilizer spreader and go to town. You can purchase lime from your local home improvement store. It comes in 50lb bags.
I also add a high quality probiotic supplement to their diet, which encourages the growth of good bacteria in the colon (good bacteria combat bad bacteria and other intestinal parasites).
There is also an old german product called Olewo carrots which are nothing more than dried pelletized carrots. The manufacturer claims that the oils in carrots combat parasites, however, I know that carrots also add bulk fiber to the diet which also encourages the growth of good bacteria. You can also try adding 100% pure canned pumpkin (not the pie filling) in place of carrots. Don't overdo it, as too much can act as a laxative, rather just add a tsp or two.
Good luck, sorry that you're both feeling under the weather! Get well soon!