Can you be in your own clan chat and also in a guest clan chat simultaneously? If so, there should probably be another tab for the chat in addition to the "channel" and "clan" chat tabs. If not, then the triple slash "///" to type in guest chat could just be the same as double slash "//" because you can't be in more than one at once.
Yes you can be in your own Clan chat whilst simultaneously being a guest in another Clan chat. You'll also have access to the current Friends chat too btw!