I don't really have any experience with Isopods, but my answer is no.
And here's why
When or if an animal gets sick from eating other animals droppings, it's not from the actual droppings that it gets sick from(it's really a bunch of unused nutrients) but the bacteria that live on it. Therefore, since the isopod has already eaten the geckos droppings, the bacteria on it has already died off due to the isopods digestive system.
Hope this helps.