Just because we haven't written up a specific ban on something doesn't mean it isn't against the best interests of the site nor that you are guaranteed to not "get into trouble" because of it.

Registering more than one account per person is certainly discouraged. We've identified several ways that are so clearly against site interest as to be worth out-right banning.

I have tye  for the same reason that I don't use 'root' to play tetris. You'll notice that with extremely rare exceptions I don't post nodes from that account nor chat from it. There are other accounts created for providing services and those who run them tend to have the good sense to not post / chat via such.

There are also some people who seem to have a hard time avoiding creating accounts for more whimsical reasons. Some of those actually do make postings or chat but usually quite rarely and only whimsically. These are tolerated with varying degrees of mirth and/or annoyance. Usually the annoyance doesn't rise to the point of any type of intervention. The behavior certainly isn't banned.

Replying (even indirectly) to "yourself" using an alternate account either in a thread or in the chatterbox is certainly against site interests, more so when not done whimsically. No, it isn't banned.

Even just keeping two accounts and using both to post is also against site interests. Luckily, the cases of this are quite rare. Most people seem to realize that such games are best kept to a minimum. Usually people who create a separate account eventually mostly abandon one or the other, thankfully.