I can think of at least two reasons for the dual login. One would be that the software is rather old, and not always the most elegant in handling things. The programer first created one type of login, went for a coffee, got distracted with office chit-chat, returned to his desk, forgot about the first login, then created the second one (ok, not likely, I admit
). Or he created the first one, and further down the road, someone pointed out the deficiencies in that one, or the need for a different one, and, by that time, it was too late to redesign the first, so he just added the second. More likely?
The other would be users who use it to login from internet cafes, libraries, work... forgetting to logout leaves the site open to the next user. Probably not with earth-shattering consequences for a photo forum, but the software was created as a general one, for many types of forums (I think).