I think requiring it could be a little touchy, but I'm not opposed to it.
I see nothing objectionable with a couple creating an account together, always sitting at the computer together when they use it, and always speaking in the first person plural. The codependency of such a relationship is another issue, but frankly none of my business.
However, I think it would be good to rule that if people are going to write any posts in the first person singular, then they must create their own account. It's up to them if they share the password, only use one account for following threads, and then just hop onto the other account to post a reply as the other partner.
It comes down to this: using the same account may be easier for the two people, but it makes it harder for everyone else. It presents a roadblock to effective communication. It wastes our time trying to figure out who's talking instead of focusing on a response. In the extreme, it can lead to people getting fed up and just ignoring these users rather than trying to help them.
I don't need labels to define me. They're sticky and I hate the glue they leave behind.