What about Bill and Arthur Weasley? Both were Secret Keepers for their houses in the last book, and Bill informed Lupin of their location and Lupin was able to come to their house.

The Potters could have done exactly the same. And saying they trusted their friends is no answer. Its as if Harry refused to use a Shield charm because he's confident Hermione's trustworthy and will place it for him. Why take the chance at all? You go to friends for help in situations where you can't do things yourself. This is clearly not one of them.

And eucatastrophe's explanation doesn't work either. If it wasn't known in that time that a person protected by the charm could be secret keeper himself, then Flitwick would have mentioned that when he gave a technical description of the charm. Surely it can't be claim that Hogwart's charms master is unaware of something like this.

There's no help for it. It is a plot hole. Doesn't diminish the series much, but I do wish Rowling had avoided it. All she had to do was make Bill and Arthur each others SK's.