I am guessing, but I can think of only a couple of relevent things:
first, When you use ssh to login, the ssh daemon on the linux system allocates a tty/pty device exclusively for your session. This is not going to be the same tty as your console. Thus, it's a totally different input/output channel.
second: I imagine the method would be a piece of code that replicated everything typed and echoed in your login session to be echoed on the console. However, that's not the same as there being an active login on your console. thus, you could see the activity, but you couldn't share a login that way.
The one or two times I have seen something like this was a messy hack. I don't actually know how to reproduce it myself.
Sorry I couldn't help much...