While there is no exact rule that prevents people from making threads like this that specifically limit the conversation to just two people, the public forums are really meant to be more of an inclusive place for everyone. Private messaging or profile comments are really a better option if you just want a direct conversation one on one.
Please remember that the entire site gets a notification when responses are posted on the public forums. It's meant to be an indictor that there is a conversation going on for the userbase to read and respond to if they feel so inclined. If your responses are really only meant for each other, then it excludes everyone else from participating.
With that being said, since this is currently not against the rules, you're more than welcome to continue with this thread. However, please be aware that Staff will be unable to regulate your request for this conversation to be limited. This means that if anyone else wants to start commenting and participating in the conversation, they are more than welcome to as well.
Staff cannot prevent anyone else from posting here, which is another reason your private and personal conversations are probably best served in another feature.
As always, if you have any questions, please let me know!