Snake, I am only aware of one couple on ASF and there is one other case of two people from the same household who post with different usernames.
These situations do crop up from time to time and I understand the implications of such an arrangement. When two people from the same internet connection both post on ASF with different usernames I let them know that they are not allowed to directly respond to each others posts in stock related threads as I believe this has the potential to compromise the integrity of ASF.
But by the same token, I am aware that these sort of situations do occur and as long as I am aware of them and that the members in question are prepared to abide by the rules I insist on, I can't see any real problem. I'm sure it happens on other forums all the time.